Qt (without QML). Has been around for ages and works beautifully. GTK is more complicated. I have no experience with libadwaita, but if in doubt I'd always go for Qt on Linux. It's stable, has great tooling and good documentation. And it hasn't changed much over the years, so most information you find - even on Qt 3 - is still mostly accurate today.
Love the idea of Plume; signed up to the mailing list. Btw the website layout breaks on very wide browsers (images in the .features-section overlap text). Maybe consider some max-width for the page.
And I also have to say that my days with Qt are quite some time ago. I never really got into QML, so if you say that QML is a great choice I'd put more weight on your word than on mine.
Qt Quick (The Qt Company's library that is exposed in QML) is very advanced today. I'm able to do things that would be considered very difficult with Qt Widgets. For example: true drag and drop between items in a virtualized list: https://twitter.com/plumenotes/status/1772599295243440137
Also, they started to work well on native widgets (at least on macOS), for example they support native dialogs, context menus, etc. Very helpful.
I'll probably write a blog post about the development and architecture (some said they'd be interested).
