If you need to write something right now and want to target the desktop, I would choose fltk-rs.
I can recommend fltk-rs because it just works, has cpu and gpu drawn widgets (your choice) and is very low on resources. @MoAlyousef has been doing great work here. The community is small, but it builds fast and link times are extremely short, and you have multiple state and event models to choose from to avoid weird Rust code.
There are incomplete wrappers around so-called "native" GUI APIs such as libui, but on X11/Wayland there are two native APIs (Qt and GTK), and Microsoft maintains Win32 and is pushing its homebrewn version of Tauri (more or less), with all pros/cons. I suppose Apple still has a native UI, but I'm unsure whethere it's actually two APIs or even two APIs and multiple backends.
Unfortunately, the web frontend migration of most, and definitely new, applications has resulted in a decline of this space. Last I looked, iTunes is HTML, as are Steam and new Windows 10 and 11 UIs. Many teams just use something like Tauri because they find it easier to hire web frontend devs, however I'm not giving up and writing my apps with fltk
I'm uncertain what GTK with libadwaita is aiming at and if they want GTK to be used outside of GNOME. Either way gtk-rs (relm4) link times are developer unfriendly. KDE, as the prominent Qt user, has rewritten most of their desktop in QML and is moving away from QtWidgets, with its own pros/cons. You can use QML with Rust easily, but not QtWidgets without wrapping the unsafe FFI wrappers in safe wrappers. Please correct me about GTK and QT where I'm wrong/outdated.
Allow me to rant:
I shouldn't rant because I don't have time for a UI discourse, but the frustration has been building up for years.
I miss the good old days where GUIs were consistent in behavior, very similarly controllable with a keyboard, and you could develop keyboard muscle memory that worked for years. The tablet and then web focus had a negative effect for usability on desktops. And of course I miss the days where contrast, outlines, depth, widget highlighting, and intuitive UIs were the norm. Because UIs are declining, many developers do more in terminal emulators, from what I gather, which is sad and a loss in productivity.
As I'm unfortunately ranting a little, allow me to say dark/light mode should not mean #fff vs #000. You know, there are so many shades inbetween that work and are ergonomically usable for more humans and their devices or environments. Neither do I need unused empty spaces of black/white on the desktop, or aggressively hidden controls.
Pro tools like in media production or engineering haven't declined as sharply.