Honestly just pick a medium sized instance with a theme that interests you. You're not expected to only post about that theme at all. The local timeline will be

Honestly just pick a medium sized instance with a theme that interests you. You're not expected to only post about that theme at all. The local timeline will be useful because it'll have a lot of people interested in -- though not necessarily posting about -- the theme. Huge instances are bad because it's like drinking from a firehose. There's too many people with too many diverse interests. Tiny instances are bad because it makes discovery hard. On a medium instance you simply browse the local timeline to find interesting people, and their boosts will introduce you to people from other servers. Which also explains why boosting is way more important on mastodon than retweeting is on the bird site. Worst comes to worst you can migrate fairly easily to a different instance. Possibly a self-hosted one once you've built a bit of a network of people to follow, since by then the discovery problem is solved.