I concur with @poVoq@slrpnk.net’s testimony :

Collabora Office […] ended up using less server resources when idle than OnlyOffice

I actually don’t know about Collabora, but I do know that OnlyOffice uses a lot of server resources and is quite hard to set up, even when using Docker Compose.

So instead, I’m currently using the desktop client with WebDAV instead.

However, you said :

CryptPad […] uses OnlyOffice client side

So, if CryptPad provides an alternative backend for OnlyOffice, then I’ll be curious to know whether it uses less resources than OnlyOffice’s official backend, and whether there are other alternative backends.

As for Collabora, I wouldn’t recommend it, nor NextCloud in general, to be honest.