Why don’t you try
sddm-git
Just some extra ideas:
- have you tried with only the integrated GPU? Meaning removing the AMD GPU from the machine?
- Motherboard BIOS/firmware update, is any available?
I have a 2700x, no igpu in this puppy to run off. I do have another 6800 and an rx580 I could try though, I’ll probably go try them out tomorrow just to see if there’s any change.
Just updated bios as per @BS86 noticing I was a bit out of date, I’m now on the most current. And my setup boots into Windows just fine with all the same hardware+bios.
@ricklinux if I install sddm-git do I need to do anything with the already installed sddm packages (sddm sddm-kcm eos-plasma-sddm-config) or any config for it? Or just install it and because it’s newer it’ll take precedence? Also, would I need to reinstall-kernels after installing?
I’m not 100% whether you need to do anything other then install sddm-git and it will replace sddm.
@manuel ?
Probably not. But easy to try… 
Well… uninstall package sddm first.
yay -S sddm-git will automatically remove sddm - its listed as conflict. No other packages need to be changed. I am also running sddm-git because it is more or less required when running Wayland.
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Well, this is a bit of a delayed update, but wanted to make sure those coming behind me with a similar issue would have the resolution (plus those who helped get the conclusion):
Swapping out to another RX6800 solved the issue instantly. The GPU works totally fine in Windows, but for whatever reason, won’t boot in Linux. Swapped the one that doesn’t like to boot into Linux to my wife’s Windows-only pc, and haven’t had any issue. Weird, but whatever.
I’ve had some other issues since that point (mobo went out, threw i/o errors and took a drive with it) so apologies for the slow followup. Thanks again to everyone for the help in troubleshooting such a weird issue.