GNOME still has some problems with fractional scaling, but KDE works perfectly. I'm using two displays, one with 150% and one with 100%. No blurry apps and absolutely no issues. Have you tried it recently?

Can you independently set desktop wallpapers on the two screens? I know this seems nitpicky but it's literally impossible with Ubuntu/Gnome as far as I know; I have one vertical and one horizontal and have to just go with a solid color background to make that work.

Yes. It was actually more tedious to do the inverse when I wanted three screens to do a rotating wallpapers from the same set of folders as I had to set the list of folders three times

I've been using fractional scaling on Gnome for years (including on the laptop I'm typing this on) and haven't had any issues. I haven't tried it with two displays that are set differently though. Is that a common thing?

KDE is in better shape than GNOME, but there are still some nits. Nearly all the available third party themes for example are blurry or otherwise render incorrectly with fractional scaling on.

So don't use a third party theme.

Problem is, the stock themes aren't to my taste at all.

Why not send a pull request to one of your theme maintainers?

To my understanding, doing that wouldn't be helpful due to hard technical limits that can't be reconciled. Most window chrome themes are Aurora themes, which don't play nice with HiDPI, and to change that they'd need to be rewritten as C++ themes (like the default Breeze theme is), which is beyond the capabilities of most people publishing themes.