There’s also secureblue, which is a fedora atomic fork with nice security hardening

This is a protocol audit. The app itself has not been audited yet, but they supposedly plan to do it.

Or GNOME Papers https://gitlab.gnome.org/GNOME/Incubator/papers (still beta)

The only apps that don’t work on GrapheneOS are apps that use the strong play integrity API. These don’t work on LineageOS either, only stock OS.