Mail and calendar I'm still trying to figure out. VPN you don't need as long as you use HTTPS everywhere.
password manager
documents
Collabora Office + LibreOffice
What are the pros and cons relative to proton?
Pros: free, open source, and 100% offline with no intermediary company. Your file security is entirely in your own hands.
Cons: you must devise your own cross-device sync system. I use Syncthing + Syncthing-Fork.
What are the mobile apps like?
Collabora is currently just bad lol. It's best reserved for really simple edits, if not just for viewing, with all major changes made on a desktop/laptop computer. KeePassDX isn't terrible but it can't view all the fields that the KeePassXC desktop platform can, and getting it to take PIN instead of password for vault-unlocking is really convoluted (although you'd only have to do it once).
What assurances do you have they won't go full proton in the future?
They're all open-source so anyone dissatisfied with the direction that the maintainers go in can fork them at any time.