on the side of our apps (or anything written with Qt) all of that (and most important seamless compositor restart/crash recovery with the application surviving) everyhting should be there for 6.0. with other toolkits the mileage might vary (depending when they include the required changes, when a given distribution packages them and so on)
We sure do plan of moving more and more of our app to the new convergent ui toolkit made with QML and Kirigami, in the future more and more of our apps should become mobile ready
to me, the new kwin tiling that first appeared on 5.27, but this will probably change, as feature development on it is about to reopen :)
just had dinner, therefore, great :D
To just test out without any risk and not touching any running system i would suggest a live image such as KDE Neon Unstable which has dailiy updated snapshots of our software stack
as moving to Qt6 from a developer POV, It has been remarkably uneventful. there are api changes for sure (Especially on the QML side of things) but the changes are not as great as say, Qt4 to Qt5.
We do aim to improve our design and usability further as much as possible, however, one of the nice things of free software is really this big choice. There are different projects and one size never fits all, if some people find the software written by our friends over GNOME more suited with their needs, that's totally fine.
It would also be interesting hearing on the motivations for this choice tough, as it always help us improving