The last versions of KDE 4 had fixed a lot of issues and were a solid DE IMHO. I had left KDE for a while when KDE 4 was first released indeed, but came back later.
They decided they would not do the same mistake of releasing such unstable versions again in the future, and indeed, plasma 6.0 was usable since day one.
Have you tried Plasma 6 or a recent plasma 5? If you liked KDE 3, you might like it. I find it's a really pleasant environment, I'm not sure something from KDE 3 is missing. It's also quite lightweight, I run it on a tablet with 1GB of RAM, it's a bit slow but quite usable. The web is pretty much unusable though for the most part unfortunately.
There's also a big work of ironing out the small bugs and they are working on making things more cohesive with a design system and some work to unify qml-based and qtwidget-based components (yes, on that front, qml was not a thing when KDE 3 was released, the UI was probably more cohesive - though Breeze, KDE's theme, already does a lot for this and is beautiful).
I don't know Trinity but that's of course another option.