I thought it may have been that kscreen is somehow conflicting with xorg.conf potentially leading to the occasional and seemingly random black screens on startup I’ve mentioned.
I’ve tried disabling kscreen from auto-starting, and writing a much more complete xorg.conf with monitor, screen, serverlayout, and device sections specifying which device with which monitor via Option "Monitor-outputname" "monitorsection", designating the primary monitor with Option "Primary" "bool" and generally following the documentation for xorg.conf files found here (I would post this config, but forgot to keep a copy of it for posting). This didn’t fix anything nor set any of the values, I believe either kscreen is still overriding, or xorg.conf is using an internal default.
I’m now back on the following configuration:
Section "Module"
# Load modesetting module for the iGPU, which should show up in XrandR 1.4 as a provider.
Load "modesetting"
EndSection
Section "Device"
Identifier "amdgpu"
Driver "amdgpu"
BusID "PCI:58:0:0"
Option "TearFree" "true"
EndSection
Side note: found that TearFree true fixed the stuttering for the external display (unfortunately can’t remember the site that I found this on), but now the internal display is choppy, perhaps something to do with PRIME and the copying of framebuffers, will need to look into this further / may create a separate topic.