Your wish is granted.

Want a whole new desktop?

https://www.trinitydesktop.org/

Want a whole new OS with it built in?

https://q4os.org/

Want to keep the KDE 6.x foundations but replace the desktop shell?

https://github.com/neeeeow/Klassik

> I wish I could have a DE that looks like KDE 3 or before...

You probably want Trinity DE then. It's literally KDE 3, forked and maintained.