They messed up when they made sub-reddits. Sub-reddits are a harmful combination of community and content. Popular content becomes a huge community, and all of the issues based on Dunbar's number happen as a result, which includes abuse of powers from the admins. Instead of using sub-reddits for content you should have tags. Communities should never be bigger than 150 people, and should not be limited to a single idea, in much the same way that Hacker News is aligned generally towards hackers. You would then build up communities of communities, where you would be beholden to your own community and the communities could only moderate outside of their community at the community level.
From there you only have to do a decent job like TikTok has done of content discovery to allow new content to thrive, and from comments on that content allow people to find better communities.
If another community is mis-filing tags, your community can apply moderation against their community such that your community does not use their tags.
The other way to do this is to have just as good support for +tags in queries as -tags in queries. When adding a tag for no reason is making it less visible to the people who are filtering out that tag, they stop doing it.
This is a bad take. You are asking for the forum and sub forum/tag approach. A community that small will simply not be engaged with frequently enough to avoid seeming “abandoned”. This is exactly the set of circumstances that led to content aggregation sites like Reddit.
Sub-forums are a usability nightmare, but reddit already solved the sub-forum issue. You would be showed content in a flattened manner, but in the hierarchy where the closest to you is shown first.
From there you only have to do a decent job like TikTok has done of content discovery to allow new content to thrive, and from comments on that content allow people to find better communities.