My number one recommendation to all new (and old) fediverse server administrators is: make your registrations closed, however your software allows for it. For Mastodon I think "registration with a review step" is the minimum level you should be locked down. Open-registration instances are

1) going to get overwhelmed with computing needs
2) likely to break down socially, you have no guarantee that the people signing up are a good fit for your existing people
3) going to host spammers