I can't speak about the grayness, but the lack of snappiness I think is thanks to the bloat and complexity of modern UIs. KDE in particular is a beast with a ton legacy code built up over the time, and a lot of bits and pieces put together by people from around the world, which results in a lack of cohesiveness... but that goes for most Linux DEs.
XFCE comes a bit closer to the old UX and cohesiveness, but is still a bit off. In saying that, Chicago95[1] for XFCE does a really great job of bringing that classic Win9X look to XFCE, so it's worth giving a shot. There's also a fork of it called MENT2K[2], which recreates the Win2K experience, also worth checking out.
The DEs I've seen being closest to recreating that classic experience have unfortunately been outside of Linux: ReactOS being the most obvious choice, and the other one being SerenityOS. Although not viable for daily driving yet, still fun to play around with in a VM.
Thanks a lot for the writeup and all the pointers. Yeah, I think incoherence might play a role here.
Will definitely check out those themes and have a look at ReactOS (what I wanted to anyway but was procrastinating)