There are several mental-health communities. A few I can think of are /bipolar and /ocd.

the user feed is just made up of posts from people you follow in chrono order.

You can also follow hashtags, block hashtags, and block words/phrases.

The Local Timeline shows posts from everyone on your instance. The Federated Timeline is a fire hose. It shows all posts in chronological order (as received by your instance).

Yes, the percentage of Twitter users that moved to Mastodon was small BUT the numbers of users was a huge increases over previous users, Yes, many instances came up and went down BUT they had small user counts. Finally, many people tried out Mastodon but didn’t stick around BUT the Average Mastodon Active Users by Month (first line chart) increases in the last period.

I’m sticking with Mastodon (and Lemmy). Besides, I know John Mastodon.