How to pick a Mastodon Instance
TL;DR: Best instances for NSFW content creators without loli/shota: https://monsterpit.net, https://mastodon.host, https://artistalley.porn, https://mastodon.social. I also looked at https://humblr.social, https://pawoo.net, https://baraag.net, https://mastodon.art and https://kinkyelephant.com, but I suggest staying away from them for now.
In the previous article, I explained the motivation for moving to a federated network like Mastodon, but actually picking which instance to be on isn't exactly easy. However, depending on the platform you've chosen, migrating to a different instance may not be so difficult, so it might not really matter too much.
When I decided to move to Monsterpit.net, I looked at a few instances that might be suitable for the kinds of content I post, and used some criteria to determine which ones I wanted to end up on. Your mileage may vary, of course, but here's what I looked at:
Registration
Most instances are open to registration, although depending on the target audience of the server and how many users are already on it, the registration may be restricted by invitations, closed completely or be tied to an account on another platform. Pawoo.net, for example, which belongs to Pixiv, is tied to your Pixiv account, so if you don't have a Pixiv account, you can't get a pawoo.net account either.
Server Location
Not all servers disclose what country they're from, but you can usually tell, from a combination of the instance's rules and other online services that try to detect what country a server is from. If the rules mention anything about not allowing Nazi content, then they're probably from Germany or France. Server location was a deal breaker for me though, because I didn't want to be on an instance that would be subject to the same US laws that would affect Tumblr's NSFW content as well. Most of the European servers don't have this problem; France or Germany are fine, Switzerland, even better. Avoid humblr.social for now though; they appear to be on a US server.
Code of Conduct
Most of the time, the specific rules aren't usually an issue, but because every instance is run by a different admin, they're allowed to have their own rules. On most Mastodon instances, there is just a standard code of conduct, with very similar wording where you just need to be nice, tag your NSFW content, and not espouse any sexism or racism. Which seems pretty reasonable on the surface, but I've seen worse codes of conduct elsewhere in open source spaces that ended up in things like witch hunts on Twitter and overpolicing of free speech. I guess we'll see what happens, but I prefer shorter codes of conduct with simpler wording. Not the monstrosity that exists on the mastodon.art instance, for example.
NSFW Content
Most instances allow NSFW content, but will ask that you tag them. A few of the instances which are intentionally designed for NSFW content won't ask that you tag your work, but might be subjected to blacklisting from other instances because of the lack of tags (and on their server, they have rules about tagging NSFW content). It's not a big deal if you're required to tag NSFW content anyways, so long as you remember to do it, since you can choose whether or not sensitive content should be hidden behind a clickable overlay.
Loli/Shota
I'm one of the artists who aren't interested in loli or shota, but there are definitely people with those interests, and there are instances that cater to them. Originally, I thought it was no big deal to be on baraag.net which allows people to post loli/shota content. The only problem appears to be because there are so few other platforms that allow it, one would expect that the platforms that do allow it will be flooded with them. This caused a problem for me because it meant I didn't want to look at the instance's local timeline, and I had to unfollow the default followers when I signed up to avoid looking at loli and shota content. It also meant that all the bigger instances, which don't allow loli/shota, blacklisted baraag.net, and a user like myself, who has no interest in loli/shota wouldn't be seen on the other federated networks that had such a blacklist, just because the instance I signed up for happened to allow loli/shota. So bottom line, if you're not into loli/shota, and you see an instance that allows it, better avoid it to be sure people on other instances can actually find you.
Blacklisting
In order to ensure good visiblity from other instances so that people can follow you and interact with you, you should pick an instance that isn't blacklisted by the major instances. Mastodon.social is probably the biggest instance on the Mastodon network. Usually, an instance will list what instances they've blacklisted, either on github or directly in their code of conduct/TOS page. Another way to find out is to search for specific tags that you see on the instance that you're trying to test. If you can see posts from that instance, then it's not blacklisted. So for example, go to mastodon.social and type in a tag from another instance you're trying to check. If you can see posts from that instance in the results, it's not blacklisted. If you're not logged into mastodon.social or some other bigger instance, you can just use something like https://[domain name]/tags/[tag] to search for a tag from that instance.
Conversely, you may prefer to be on an instance that doesn't blacklist other instances, if being able to see all kinds of content is important to you. However, finding out what that instance blacklists, if it isn't mentioned at all, is even harder than finding out if a specific instance is blacklisting it.
Character Limits
Another advantage of Mastodon is that each instance is separately configurable. Because it's like Twitter, Mastodon also has character limits. However, the exact character limit is determined by the administrator of the server. Some instances will mention explicitly that they've changed it; others won't. If it's not mentioned, it's probably safe to assume they're using the defaults (I think it was 512 characters per toot).
Now that I've explained the criteria, here are some of the ones I've looked at:
The two that I like the most are monsterpit.net and mastodon.host.
Monsterpit is specifically for teratophilia and other monster type content, but they're pretty free about what that is. I've seen vampire and werewolf things in there, as well as more alien looking monsters. They have a "Be Excellent to Each Other" code of conduct, which is pretty simple and seems reasonable, as well as a 6666 character limit for toots. They're also hosted in Germany, so you don't have to worry about what will happen to your NSFW content.
Mastodon.host is more general, but is also itself pretty interesting. It does have some minimal rules about tagging, and just being nice, but nothing too crazy or oddly specific. What's interesting about it is all the other platforms and features it offers you when you sign up and the instance is set up specifically to not blacklist any other instances. If you're looking for something specific, even if you don't end up signing up to that instance, you can still use it to tag search for pretty much everything on the entire Mastodon network. It also has a 2048 character limit for toots and 512 for profiles. It's hosted in France, so don't worry if you plan to post NSFW content.
Mastodon.social is probably the biggest Mastodon instance around. It's nothing too special, and has a standard code of conduct. But it's not a bad place to be on if you plan to post NSFW content, as they are hosted in Germany.
Artistalley.porn - Another one for NSFW stuff. It generally seems like a nice place, and you won't have issues with it as it's hosted in Germany.
The instances I've mentioned so far do not allow loli/shota content. The ones I'm going to mention next either allow it, and are therefore, blacklisted by mastodon.social or have other issues, and I cannot advise signing up for them just yet.
humblr.social appears to have been made as a tumblr replacement. It's specifically aimed at NSFW content, has a minimal code of conduct and appears not to be blacklisted by bigger instances just yet. The only problem with it is as far as I can tell, its servers are hosted in the US, which means it may end up having the same problem as Tumblr in the near future. If this changes though, then it wouldn't really be a bad place to post content on. I'd probably still prefer artistalley.porn though, if your content is illustrated, and not photography.
pawoo.net is Pixiv's own Mastodon instance. You'd need a Pixiv account before you can sign up to use it, and it doesn't appear to have much of a code of conduct (not one I could read anyways), but the biggest problem I have with it is that like Pixiv, it allows loli/shota, and bigger instances have blocked media coming from it. Even if it weren't for posting loli/shota, I'd still have an issue with it because their servers are in Japan, which means you can't legally post uncensored NSFW content on them.
baraag.net - I was on this instance previously because I heard of it from a different post on Tumblr. I kind of joined it first without really realizing what the implication of being on a loli/shota instance would be, and while it's otherwise not a bad place, it wasn't good for my visibility because it's been blacklisted by so many other bigger instances.
Mastodon.art is already closed for sign ups anyways, so if you had any hope of being on it, that ship has sailed. For now anyways. Despite the closed sign ups, I still wouldn't join it because their code of conduct is oddly specific and restrictive.
kinkyelephant.com is aimed at photographers. It doesn't look like a bad instance either, as they've got a minimal code of conduct and are hosted in France, but their sign ups are currently closed. I'm not entirely sure if their rules about no child porn applies to illustrated works either, although that may just be because it's for photographers and not illustrators.