I am pretty sure it's a power issue. I'm in a shed, where the air conditioner turning up to compensate for heat outside is enough to make the lights flicker.
Heat makes it worse, since the AC and the PC have fans at full blast. It was doing this after a full internal cleaning, so that's out. I'm unfortunately not nimble enough to extract the PC from its location and clean it again right now, as it's approximately twice the size of a traditional PC tower.
Turning my game settings down to potato mode reduces the crashing, so I suspect the GPU running on full definitely exacerbates the issue (lending credence to the power being the problem) there's nothing in the windows event viewer beyond unexpected power loss, and no extreme temps being shown in SpeedFan.