It is my opinion that anyone who considers Qt6 to be "bleeding edge" at this stage of the game is probably deluding themselves. Qt6.2.2 was the first to be feature consistent with Qt5, and we are now up to Qt6.7.2. Qt IS moving a bit too fast, but Debian is also way too conservative in my opinion. Stable doesn't have to equal "Old". I can't remember the last time an update to my Arch system (with way more recent versions in their repos) caused any any real problems. *shrug*
Anyway... you don't have to update your system if you don't want to. And you don't have to build Qt. Check out the beta thread starting
with this post.
And yes ... Noble has Qt6.4.2. They missed the boat by days to match the latest Ubuntu LTS with Qt6.5 LTS, but Noble should support building Sigil 2.3+ out of the box with repo-supplied dependencies.