Zrythm is an interesting open-source digital audio workstation (DAW) software package. It's been making use of the GTK toolkit but now the developers have decided to switch to Qt6 instead.
Switching the UI framework sounds like a massive refactor.
Qt is by far the better framework. This could also be a chance to implement a super UI/UX, it could also be a complexity hell. I will have a look at the project, let's see if the outcome wil be a better product.
Not on Sway, every Qt app i've used feels like it tries to put in every option and dialogs always look too busy, GTK is quite abstractive and opinionated but i'll deal with that
I can recommend trying it. Used it on my MacOS installation. Free version works great, but I felt the need for some paid features sometimes (more convenient workflow). But you can totally get job done with the free one just fine.
It's really rare for a project to completely rewrite to a new toolkit. VLC in circa 2007 did it (moved to Qt - even stole their volume control widget directly from Amarok at the time). GCompris ended up as a KDE project despite originating in Gnome (along with toolkit change, but it weirdly kept the name). LXDE->LXQT also. But I don't actually have that many examples.
They are C++ already. If they deigned the application well, a UI toolkit change should not be too bad. Not trivial but manageable.
I always hate to see apps move off GTK but their first point is about cross-platform and there is just no denying that Qt has a vastly better cross-platform story than GTK does.