Wayland is the only place that HiDPI really works correctly, so I'm surprised to hear this.

Scaling just works... except with XWayland apps. And I think we should investigate sommelier more for that, though it's increasingly irrelevant to me, as a user. Note that Chromium seems to be a big burden right now, as Ozone/Wayland is not ready yet and is not included in default configurations. (But I'm going to start publishing Chromium+vaapi+wayland builds to nixpkgs-wayland, which I maintain).

Let me know if I can answer questions or help. Sway is the first place I've finally get decent about Linux+HiDPI.

edit: To emphasize my point, I can actually step up/down the scaling factor in .1 increments (imagine Ctrl +/- keybindings). It's not quite as smooth as ChromeOS's entire UI scale, but I honestly never thought I'd see this in Linux.

It should work perfectly with native Wayland apps.

However, Xwayland apps will be blurry. This is a harder problem to solve, but KDE is working on a solution which would work for sway too.

Writing this comment on a 4K display on a mixed-DPI desktop. Works great!