Yes, the current release is based on GTK4/libadwaita but current development already switched to QtQuick. See this post for details.
Besides temporary freezing when saving projects or performing undoable actions, those are most likely GTK bugs or missing functionality (the backtraces don’t lead me to believe it’s bugs in Zrythm, and those operations work well on GNU/Linux). No point trying to debug GTK issues IMO - I reported a lot of them, some got fixed, some are on the backlog. Better to focus on the Qt6 port instead.
Besides being a pain to build (dependency on Rust for SVG support, GNU/Linux-only libraries like gettext and appstream for libadwaita, the meson build system itself regularly having bugs on Windows/Mac, etc.) GTK4’s support on Windows and Mac is poor in my experience. A lot of people find Zrythm unusable on Mac and Windows - even for basic operations. I saw you’re having issues on Windows - on Mac things are even worse. To be fair, it could be my fault, but the backtraces I see usually point to bugs in GTK.
The GTK maintainers don’t really care about platforms besides GNU/Linux (dare I say besides GNOME). The general approach about non GNU/Linux platforms seems to be “if it works it works, and if it doesn’t fix it yourself and send patches”. GTK4 has a very nice and well documented API so if you are developing an app for mainly GNOME systems and you don’t care much about cross platform or cross-DE compatibility, or if you are willing to maintain GTK’s Windows and Mac backends yourself, GTK4 is a great choice. But if stable Windows and Mac support is a requirement for you and you don’t want to waste your time as an app developer fixing low level toolkit issues, I wouldn’t recommend it.
Of course this is just my opinion/observation, please don’t take this as a fact.