The free app BetterDummy
resolves the resolution issues on M1 Apple Silicon, with one major drawback:
You lose HDR support
Thankfully it's easy to toggle BetterDummy as needed, but that's the current situation. It's due to a limitation behind the scenes with the macOS Quartz virtual display mirroring.
I was able to configure dual Pro Display XDRs at 3840x2160 HiDPI on a 14" M1 Max.
six_montahue
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