Last time I've looked at Relm4 it was in the middle of global refactoring that required changes to all codebases. Is it stable(-r) now?
Also what is this cross-platform you speak of, if we're talking about Gtk4? Compiling glib on Windows doesn't sound very cross-platform to me. Heck, Gtk4 doesn't sound very cross-platform if we're thinking about anything that's not Gnome.