First, what do you mean by "C is just so old"? That seems like a language problem, not a GTK problem. Tbh, when it comes to documentation, you're likely better off with C as the official GTK docs targets the C API (https://docs.gtk.org/gtk4/).
Also, what do you mean by "it doesn't translate well to Rust"? Because, Rust, like other supported langs like Python, have bindings that are equally well-documented to an extent. I haven't used the Rust binding but I've used the Python binding extensively and there are references to all the APIs (https://lazka.github.io/pgi-docs/)—same with Rust (https://gtk-rs.org/gtk4-rs/).
Lastly, I can understand not using GTK for cross-platform apps, but not for the reasons you mentioned. While GTK's primary target is Linux, you can technically still make it cross-platform.