Linux

I haven't ran this for a few months, so I may not remember everything I had to set up.

Anyway, it's just regular install of Proxmox that I then installed KDE Plasma on top of (same as with plain Debian).
I had to remove WiFi interface from /etc/network/interfaces for it to work as expected.
I also set up NAT for guests (10.0.0.0/24) (lower middle in screenshot).
To guests I added vhost-vsock-pci to make Vsock work (used for Waypipe), and VirtIO sound card to make audio work through host. (Under args: in middle of screenshot). You can see audio show up in Applications tab of sound menu (bottom right), currently playing music from Debian's Firefox Window (top, slightly left).
Waypipe is set up to automatically listen on host using KDE's Autostart menu (bottom, center). Applications are launched using desktop shortcuts which send a command to guests over SSH (middle, slightly left).

One thing you may notice is app icons not working (Wayland icon is shown instead). I don't know why, it only works for LibreOffice. But that's just cosmetic.

End result is that aside from guests not having HW acceleration for thing like video playback, things just work like with normally installed programs. Double-click an icon, and it launches.

I haven't set up anything nicer for storage. SSH is sub-optimal due to useless encryption, but for the occasional 5MiB file this doesn't matter.

One thing I am not sure how to do "properly" is Waydroid. I tried to set it up in unprivileged Arch LXC, that did not work. I could just put it directly onto host, but that feels wrong. And inside a VM I can't use GPU acceleration, which even on a more powerful machine was a quite painful experience.