Two main options for self-hosting:
- OnlyOffice - focus on compatibility with MS Office formats, but full disclosure, it has ties to a Russian org (R7? Product is marketed as R7-Office), but the primary developer seems to be Latvian (Ascensio System SIA)
- Collabora CODE/LibreOffice Online - Collabora CODE is a community edition of a for-profit office suite based on LibreOffice (maintained by The Document Foundation) by a company in the UK
Both have integration with NextCloud and probably other cloud suites (I think Seafile supports both?), and I think they can both be used standalone. OnlyOffice also has client versions that can be used separately or with a cloud version.
Some technical differences:
- rendering - OnlyOffice renders in the client, Collabora renders on the server and sends images back to the client; both support collaborative editing
- format support - OnlyOffice focuses on MS Office formats, LibreOffice focuses on open formats, though both have support for multiple formats; you'll likely get more rendering differences vs MS Office w/ Collabora, though both have their issues
- features - OnlyOffice has limited features, Collabora has whatever features LibreOffice has (read: lots)
I personally use Collabora, mostly since I use LibreOffice anyway, so I'd rather deal with some jankiness w/ the server-side rendering than deal w/ jankiness due to MS Office support. I use it through NextCloud, though I'm trying out Seafile and may swap to that instead.
Editing spreadsheets on mobile works for me, but it's janky. Not sure how the OnlyOffice experience is though. Fortunately, I mostly just use it to edit a couple cells pretty rarely (think budgeting), so it gets the job done.