Most things, if not available as flatpak, can be installed inside another distro on distrobox. It runs in containers, so things can access a root filesystem (Just not the main SteamOS one), and is a pretty seamless experience, once installed. I have a bunch of non-flatpak software running that way, and it works great.
See https://www.gamingonlinux.com/2022/09/distrobox-can-open-up-the-steam-deck-to-a-whole-new-world/
Onlyoffice does now have an ARM version, I run it on a raspberry pi 4 (integrated with seafile), and it works fairly well. Can't vouch for how much power it needs, to say if a pi 3b+ will be enough, though. Pretty sure it's lighter than collabora, since more is done on the client side.
I agree with the other commenter that suggested cryptpad, though. If all you're after is a Google docs like collaborative experience, cryptpad is brilliant, and much more resource friendly. (The office editor it uses is also a slightly modified Onlyoffice, so almost exactly the same feature set)