Honestly, the big issue is that Gtk is used as basis to obtain the scaling factors for all browsers, Java, and several other tools including everything based on electron.

As long as Gtk refuses to support this, I can fix KDE, but I can't fix Java or browsers.

Java, Firefox and Chrome reject patches, saying Gnome is responsible. Gnome says they don't care.

And I don't want to run custom builds of Firefox and Java anymore. It was such a hassle constantly building them from source with my old patchset.

I think if KDE actually had a working implementation of this then that would go a really long way towards convincing those other things to support it. IIRC Chrome at least doesn't get the scale from GTK. But KDE's Wayland implementation is still unstable (in part because it seems to have a lot more features than GNOME's wayland implementation) so I think it will be a while before that happens too.

If you are really committed to using Linux and don't want to wait then I said it before but I think a better use of your time would be to spend a few hundred dollars on some different monitors that can work at just 2x scale, and then let someone else deal with this.

I've got a 27" 4K HDR10 DCI-P3 1000nits monitor with builtin KVM.

Usual price point around $1600-$2200.

Comparable 2× monitors start at around $4500, and the cheapest option only supports macOS and has a $1000 monitor stand.

A "few hundred dollars" won't be enough.

At the moment, I just work all day at ~10-15fps using Budgie with fractional dpi on X11, but it's definitely not a great experience.

I used to be 100% in on KDE, but as you mentioned, Wayland support is limited.

Budgie is atm sadly the only option for multi-monitor hidpi that's not using gnome's broken top bar concept.

Ah okay. I think the work required to get this implemented across the stack will still cost a lot more than $11,000, just saying. Also in my opinion 4K monitors at that size are an unfortunate purchase because the PPI is not high enough to look crisp at that viewing distance. Apple upgraded theirs because you need to get above 200 PPI range in order to have it look comparable to print. For me personally, I couldn't justify spending my spare time working on that when other monitors exist that don't have that problem.

4K at 27" is an unfortunate resolution / density. I know, I also have such monitor :(.

That said, 10-15 fps on X11 is a pathological case, something is wrong there. When I ran X11 and played with xrandr, the slightly larger framebuffer had zero impact (outside games) on AMD Vega64.