I'm not sure if you're familiar with the extent to which people altered things, such as https://en.m.wikipedia.org/wiki/LiteStep

Speaking off shell replacements: Are there any worthwhile ones today? I know Cairo, but it's "meh". The windows default shell is just buggy and glitchy as hell (even though I generally mostly like it - a version with fewer glitches would be enough for me, really).

AFAIK: No. These days the default Windows WM seem to dominate. I don’t know of any viable alternatives to it.

I did run LiteStep actually, it's still not nearly as much customization as what is possible nowadays with modern DEs.

LiteStep is amazingly customizable and versatile, but it doesn't come close to being able to, for example, define pixel shaders in a theme, or being able to easily change any UI element anywhere using simple HTML/CSS, or completely switching paradigms by just changing around a few dotfiles.

Not even DE. Fluxbox had literally thousands of styles. And fvwm powned ANY DE and Windows OS.