There are lightweight electron alternatives - HTML/CSS engines with a scripting DSL. Sciter is a paid option but I know there are free options depending on your language of choice
I’ve been working on a cross platform music player and went with GTK over QT because I preferred the Rust bindings, QT seemed to have a lot of non UI code I didn’t need, and I was skeptical of dealing with QT licenses (but a lot of people seem to work with it just fine - I just didn’t put the research in to see if it was right for me).
Compiling linux / mac was very easy for me. Cross compiling linux -> windows via mingw64 was also pretty easy. I had some trouble compiling mingw64 on windows, but that was due to SQLite / MSys issues that I never really figured out.