OK, so after a bit of poking at it:

1. I agree. The OnlyOffice mobile Android app (called Documents) is a much better mobile spreadsheet viewer/editor than Collabora.
2. What’s even cooler is that the app works with Nextcloud as a cloud backend. So I can log into my existing Nextcloud instance and get the benefit of the better sheets editor on my existing files with no extra work at all!
3. They say that OnlyOffice supports markdown as of version 9, but I think they mean the broader platform itself, not the Android app. For example, you cannot create a new .md file from the mobile app, and if you try to open an existing .md file, it displays a “wrong file type” error, but it does successfully open it as a .docx.

In any case, since it works with Nextcloud, the app, out of the box, is already a more functional mobile spreadsheet editor. That’s a big win in my book. Thanks!