Sounds good. Maybe some sort of forum system, like https://github.com/hometown-fork/hometown (activitypub-based) or https://karrot.world or even an instance of lemmy ? Nextcloud is good as it has a lot of plugins for sharing photos and chatting etc. Video conferencing It would be worth looking at something like https://www.freedombox.org/ as it’s all self-contained, and might be easier to set up. at least as a proof of concept?
There’s an old thread about someone doing it here: https://diysolarforum.com/threads/replace-apc-ups-sla-batteries-with-lithium.25424/ but sadly he doesn’t give any details, or any updates.
It’s wierd. APC do a really crappy small one for telecoms cabinets, but none for servers. I would love to replace my APC batteries with lithium ones.
Let us know what ones you chose in the end.