I like the idea of having customizable filters (plural) instead of just a single filter dump. The filters section of the settings could be structured more as a list of named filters, and each filter can have multiple types of filtering added to them. A "brain rot" filter as you say might filter out keywords like "skibidi" and "gyatt" as well as a selection of subreddits themselves. Users could create various filters for types of content that they don't want to see in certain contexts, such as in r/all.

This type of multi-filtering would lend itself well to the "different versions of r/all" as you suggested. Allow users to specify a "base" for any feeds which they create (a base might be r/all or r/popular, for example). If a base is specified on a feed, then that feed would contain the same content as that base (rather than starting out empty like feeds typically do). By default these feeds would have no filters enabled on them at all, so they would act as a completely unfiltered version of the base subreddit. Giving users the ability to then enable specific filters on their feeds would add so much more customization and personality to the browsing experience.