I think I had problems with the Flatpak as well. I think they have a deb and a rpm package. Maybe try one of those. Or tell me the distro you use.
Edit: I think the snap also worked as expected
Onlyoffice seems to work for me. Which packaging format did you use? (I am using the binary from the AUR) Maybe using a different one solves your problem.
It could be a stupid way of achieving this, but I just disabled the preinstalled launcher and now it launches the F-launcher automatically since its the only app that works as a "home screen". I did this via ADB
And I thought that my school is weird. Well, maybe I should reconsider that
Could you please elaborate why Brave isn't open source? The source code is on github and there's even an Brave-Git package in the AUR.