Selfhosted

I will add that if you have open registrations you will be a target for spam and trolls, and if you don't take quick action then some other instances are likely to defederate from your instance.

This depends on the instance, some will have a low tolerance and defederate pretty quickly, some instances will defederate temporarily until the spammers or trolls move to a different instance, and some won't care. But you likely won't know it's happened unless you notice you aren't getting content from that instance anymore.

One other thing is that if you're going to run an instance and aren't already on Matrix, make an account. It's how instance admins tend to keep in contact with each other.

Run the software that scans images for CSAM.

Which software is that?

If your instance freely hosts whatever without any oversight, word will spread and all of a sudden you’re hosting all sorts of bad stuff. It’s not technically illegal if you don’t know about it, but I personally don’t want anything to do with that.

Yeah, this is my primary concern. I'm hoping that there are established best practices for handling the majority of this sort of unwanted content.

If you have open registrations you’ll be doing a lot of moderation for spam.

Perhaps Captchas are sufficient?