IIRC KDE committed to never break users again like they did with the first releases of KDE 4. It's expect the first releases of Plasma 6 that the distros will actually ship to be stable, and to be mostly a Qt5 → Qt 6 upgrade. You should be able to run a stable version of Plasma (5 or 6) in any case during this period. KDE 3 to 4 was a disaster; last versions of KDE 4 were rock solid. First versions of Plasma 5 were a bit lacking but rapidly became very stable and usable, and you could actually keep using KDE 4 in the meantime. I expect the Plasma 6 transition to be even smoother. I hope I'm not wrong.
But otherwise I agree, Cinnamon seems very good and I tend to recommend it and pick it for people who I install Linux for.