What is the safest app to open Word and Excel files on your smartphone?

I’m looking for an app that can edit Word and Excel files under a mobile OS environment called GrapheneOS. We value privacy and security, so we want to be open source and do as little privilege as we can. Do you have any good apps? By the way, PDF uses Secure PDF Viewer created by Daniel Micay.

Ive used LibreOffice before aaaaannndd its painfully slow. Its available from FDroid. It uses little priviledges and is open source. Its not good, but its available.

The next best thing is probably through the web with Google Docs but we know we dont like Google at all.

There is alao collabora and nextcloud but i dont know how mature those are.

EDITs: F-Droid is compatible with Graphene, right? The document edit function is still on beta unfortunately.

The best one is Microsoft office. And it probably has better privacy policy than Google Docs. I have tried Softmaker office, it’s ok. Now I use integrated Onlyoffice from Nextcloud app

which is better, Only office or Collabora office?

I dont think softmaker office works on android.

On a separate note. Softmaker works fine and looks even better than libre but apparently I cant save to open document formats which is a big no no if you want to move away from closed source formats.