Linux

KDE is the correct answer. I guess you could also learn coding, and then any piece of open-source software would become "customizable" to you...

Install them both and try. 🤷‍♂️

I don't know if it was ever true that one was more customizable than the other. Maybe back in the early days of GNOME Shell or GNOME 2 you could say KDE was more customizable but right now it doesn't matter. KDE has like 451 million options ootb while GNOME has approximately 451 million extensions so it really boils down to which one you like more.

You very likely want KDE. It's got the most features out of the box, each of which is something you can customize (whether you want it enabled and with what behaviour).

There's arguments to be made that more minimal desktops, like Xfce, or even a DIY setup using a window manager, are simpler to customize, because they have fewer features.
For example, the panel theme in KDE involves various colors and icons and whatnot, so you need a full-fledged theme package to customize it, even if you 'just' want to change the panel color. In Xfce, the panel color is a simple hex value that you can just set.

GNOME is a weird one. It also has relatively many features, so it does still have relatively many options. Some configurations are relegated behind extensions or a separate settings program (GNOME Tweak Tool). And yes, you can technically apply CSS. But then for some relatively basic things, you need to dip into CSS or other workarounds.
I don't know, if it's still the case, but a few years ago, I just wanted my windows to not have a titlebar, which took me three days to figure out how to do, eventually solving it with CSS.
A few months later, I switched to KDE and there it can be done with a GUI setting, which took me less than ten minutes to figure out.

https://www.enlightenment.org/

You'll spend lots of time customizing it, I guarantee it.

For me the main difference is that KDE wants to be customized, whereas GNOME does not. You can theme gnome, but they don't like it and will not make it easy. As for css customizations, I themed some gtk (not gnome) apps and really don't want to do it again. The css is some weird flavor that doesn't support half of css features and you can't edit the "html" so targeting some elements is really difficult.

I wonder how Hyprland fares for overall customization