how is this different from http://www.collaboraoffice.com/ ?
Collabora works on mobile devices and integrates with Nextcloud
note: they use WASM, so the load is on the client. Collabora relies on a server part, though easy to host.
COOL (Collabora Online) requires some extra infrastructure. See this previous post.
https://news.ycombinator.com/item?id=42251940
I use COOL every day. But I haven't set it up on my low end home server yet, because of the extra resources needed.
I am aware of the additionals, like COOLWSD. But since they run this in their Android apps, it seems this can already be Pi-ready