And just like that, the issue which started this all (frame skipping being laggy) has vanished.
No re-encoding / remuxing required.
Must’ve been the AVI container of the Mobius footage (or its contents - PCM audio?) tripping Shotcut out somehow.
Arrow-keys frame skipping “works” TM fine now (no lag) with a mixed timeline of:
(1) raw Mobius footage in the .mp4 container, (with HDR on and all other settings maxed); and
(2) whatever variable (nominal 30) .mp4 footage my phone shoots in 1440 and 1080p modes (depends on which camera is used).
Thank you for your time people, I’ve learned a lot about:
(1) my machines; and
(2) my footage.
If anyone is having issues on low-end hardware, they can at least start debugging by knowing Shotcut 21 is usable with 1440p and 1080p footage - no proxies - (with the caveats of my specific footage, setup and workflow expectations) on 2nd gen i3, integrated HD 3000 graphics, and 8GB ram.
I can now happily edit, in sync, on any machine I have available. That is a massive bonus to me as a hobby YouTuber.
Thanks again, it’s been a fantastically enlightening discussion