I highly recommend Qt for this purpose.
Gtk has been a hot mess for the past few years with Gtk4 and libadwaita shenanigans. However, Qt has continued to evolve incrementally without dropping any backwards functionality.
It's incredibly full-featured and has amazing tooling. In addition to Qt Creator you also have the CLI tools for packaging and whatnot and KDE has Kirigami for components.
The nice thing is that Qt applications look pretty good in native environments like plasma, but they also look pretty good in Gtk environments, including older Gtk-3 desktops and newer Gtk-4 desktops. So you cover all your bases. You also get Mobile and Windows/Mac, if anybody cares.
The two real downsides are:
- GPL
- C++ only (ish). There's other bindings but they will not have the same level and quality of tooling. Gtk has less tooling overall, but MUCH more high-quality bindings.