Sailfish OS looks nice. but i am not sure if their strategie for applications is the way to go. Applications need to be specifically built for Sailfish OS with their IDE which uses Qt.
That means you can't run just any linux software on it.
Nope. You can use SDL2, which is behind the Godot port for Sailfish (3.5 still). Supertuxcart, Openlara a bunch of games stuff is viable. There is an active Lua Love porter and and and ...
You can run any CLI software in the terminal emulator ;) You might need to build it, if it is so rare that no binary is readibly available.
Yeah, with GUIs thats a different story...