Godot 3.6 got finally released and it supports exporting arm64 builds for Linux now. That eases development of games and other applications with Godot for the Librem 5 significantly.
All you need to do is using the GLES2 renderer of Godot 3 because GLES3 might not work properly. From my experience performance is pretty solid in case of 2D games.
I’ve now tested building a game for Android and mobile Linux to compare both platforms.
Android requires manual configuration of permissions, installing the SDK, accepting license terms, setting up a key store, signing the application… and after all those steps, it finally gets installed on an Android device and runs.
With the Librem 5 I only needed to select arm64 as architecture and hit “Export”. No changes required, no additional steps.
Only thing you want to keep in mind is implementing touch controls. The ones from Godot itself only work on Android because Wayland support wasn’t backported to Godot 3.6 and touch input will therefore only be recognized as mouse events. All of the Godot UI widgets work though and if your game supports controls via keyboard, you can actually play via squeekboard (maybe even offer an extra layout).
Either way it’s not difficult to implement some custom buttons to allow touch screen controls. I recommend adding a setting to enable/disable them for a desktop with proper keyboard or gamepad (could also apply to your phone of course).