That is a tweak for SteamVR to reduce blurriness, mostly when running with PP on. It’s not related to your stutter problem, but you can search the forum for “maxRecommendedResolution” for more info.
I mostly play Elite Dangerous on my 8K, but I do play Assetto Corsa, and I’ve not seen any stuttering, but the reported complaints mostly seem to involve the 5K+. Oddly, my current framerate in ED is only ~60 fps according to fpsVR, while my refresh rate is 80 Hz, but my in-game experience is very smooth. I have an i7-8700K, 32 GB ram, and an RTX 2080. Like you, I have an ASUS motherboard: the TUF z390-Plus Gaming wi-fi model.
Honestly, the following is a list of long-shots, but you’ve ruled out most everything else.
Go to https://www.userbenchmark.com and download and run their free tool. It has been helpful to discover PC issues in the past (like mis-configured RAM).
In nVidia control panel, try setting Power Management Mode to Adaptive. Prefer Maximum Performance can cause framerate fluctuation, when thermal management kicks in.
Have you removed everything possible from your USB ports? It might even be worthwhile to unplug your steering wheel (just as a test). It’s possible there’s some sort of USB bus contention.
Try plugging your headset into different USB ports, sometimes these are shared internally, so some work better than others. This has been a particular problem for MSI motherboards.
In your racing game settings, try setting Performance to Max Frame Rate, as a test.
Try setting PiTool Render Quality to 0.75 or even 0.5. On my 8K, I’ve found that 0.75 with SteamVR at 100% yields the best clarity of text in Elite Dangerous.
In PiTool, turn on FFR with Conservative mode. That significantly improves my framerate in ED, even though I don’t leave it enabled due to shimmering at the edges of the view.