Alright, the DB shenanigans are done.
1. Purged remaining Announce activities from standalone relays, which amounted for about 1.8 million rows.
2. Purged posts (Note objects) with no replies from bots at bae.st, milker.cafe and nicecrew.digital. Don't have full numbers, but I think it was around 3 million rows.
3. Ran a query to purge orphaned activities referencing those newly deleted objects (and maybe some others, don't know if there's any). This took almost three days, but amounted for 3.3 million more rows.
4. For a good measure also deleted empty remote actors, around 250k total (Mastodon often spams deletes over the whole network, which prompts the instance to fetch deleted users even if deletes themselves are rejected; this likely isn't the only reason they federate, but sure is more notable one). Also deleted like/repost/react notifications and all other read notifications older than three weeks.
5. Unrelated to DB maintenance, but I patched frontend so that it would make background requests much less frequently, which might also improve performance and reduce the amount of 500s.
Sadly I didn't bother getting any numbers before doing all that shit, but I suspect there's 15-20% decrease in total DB size. Right now it takes 59.3 GiB on disk.