The only real consistency i've seen between toxic communities and healthy ones is size. When you try to call 100k people a "community", it's a bad community where the toxic people have a louder voice than the good ones. If the community is a core group of <1000 participants and maybe 10000 spectators (following the 90-9-1 rule of online communities) it can be good. When it grows larger than that, it needs to be split up or shut down.

The only way i see to save reddit is to set a maximum size for a subreddit, and shut down or otherwise isolate every subreddit that grows bigger than the maximum threshold.