I've got the same issue as you. My current approach is first educating myself on how to set it up. Then contacting the mods via modmail. Should they not answer I'd make a post in the sub with my offer to set it up and then transferring the mods to the old team.
If the community accepts the transfer is on a whole different page. Lots of comments I read might use this opportunity to quit social media for good. Some communities had great value to me, I hope they'll further exist and ideally over here.
I don't see too big of a problem in copying and referencing to a preexisting community wiki.
This whole reddit thing made me start to think about Instagram again.
So let me pitch:
What about an Instagram like social where the algorithm regularly posts older pics/posts of your friends and let's you circumvent the fomo feature of stories with like an option to "snapshot" so every other user can only see it once.
I haven't looked into pixelfed yet. But would def be interested if there's interst in such an app.