Wayland supports it (and Chrome supports it very well) but GTK does not. I run my UI at 200% scaling because graphical Emacs uses GTK to draw text, and that text would be blurry if I ran at my preferred scaling factor of 150% or 175%.

This works with GTK for me at least. I've been using Gnome+Wayland with 150% scaling for almost 4 years now, and I haven't noticed any issues with GTK. Actually, my experience is essentially backwards from yours—anything Electron/Chromium-based needed a bunch of command-line flags to work properly up until a few months ago, whereas GTK apps always just worked without any issues.

GTK uses Pango/Harfbuzz and some other components to draw text, all of which are widely used in other Linux GUI stacks. GTK/GDK do not draw text themselves, so your complaints are not with them directly.