Quick follow-up, because tonight I installed EndeavourOS on my Surface Pro 4. (It's not Garuda, but it's still Arch.)
If you can follow instructions and copy/paste Pacman commands, you can install the Surface kernel. I did hit a couple of unexpected errors along the way, but the error messages were very specific. So it was easy to resolve them.
The instructions page is written very well, and there's a whole section dedicated to Arch.
There are only two things I haven't done yet: set up secure boot, and enable Bluetooth. Both of those things are pretty well documented, I just haven't tried to do it yet.
Yeah, I'm sure Bluetooth is just from something I missed. Some config I need to update, or something.
Touchscreen and pen both work perfectly with the new kernel.