The wire protocol itself is so ridiculously cross platform its almost obnoxious, there is not 64bit types by default and everything is serialized to 32bit words

The fact that bsd cant figure out its own API like DRM is bsds fault, implementing DRMs api is the logical approach since they don't have the manpower or expertise to draft their own API. The entire DRM, mesa, and set of drivers in the kernel is dual licensed MIT so BSD can use it just fine. a lot of it even has HAL stuff built in like amdgpu.

A wayland compositor doesnt even need to use DRM. If you can run vulkan WSI then you render the vulkan image offscreen then your compositor calls whatever drm equivalent api to display images to the screen. The biggest compatibility thing would be that every app uses drm syncobjs and dmabuf so those are the practically mandatory things to have working for other OS's

So its pretty clear that the issue is lack of developers for the bsds than anything else

Also udev isnt needed either... Neither is libinput. My compositor doesn't even use libinput because I found out libinput has a statistical inaccuracy bias that rounds mouse movement in a way that can compound to multiple pixels of error in a way that's noticeable. This stuff is not rocket science. Like did you know libinput tries to normalize mouse dpi by looking it up in hand written quirk files, but practically falls back to 1000 for nearly every mouse, but theoretically if you could easily input your dpi you could use a high dpi (4000+) mouse without setting your sensitivity in games to absurdly low values? This software isn't perfect and libinput did not invent basic calculus or the homer simpson rule or mouse debounce or hysteresis filters.

udev doesn't even do much man it just gives canonical names your devices and has some database of hardware overrides, like seriously I promise you guys the 30 libinput quirk files do not matter and even if they did reimplementing these things can be done insanely easily. I don't know if the hwdb thing udev has matters that much but if it does it's still not rocket science to make your own implementation that uses hwdb.

Oh and GBM isn't really needed either, generic vulkan buffer management can work fine too. My compositor doesn't use gbm.

and logind also is just dbus slop that barely does anything important, seatd abstracts everything away in a cross platform way. Even seatd is doing too much, once VTs are destroyed by tomahawk missile a "seatd" daemon is then just a proxy that hands out fd handles to input devices as a privileged daemon so that your user doesn't need to be in the input group or have /dev/input permission or have permission to directly own the drm master block device in /dev/dri/.

Basically you guys are all just lazy complainers who don't even know what these software pieces are doing you cant blame the freedesktop group for your own development failures