Alright,
I have done everything you mentioned again and beyond.
If I stop my animated directional light (night/day system) the problem stops. So I know the issue is with lights animated movement and some sort of shadow invalidation. (I’m not using virtual shadows or lumen)
But even with all post process materials disabled, shadow cascades at max, dynamic shadow distances checked (no significant improvements at any lower ranges), resolution at maximum, shadow bias, light source angle and intensity, auto exposure settings, skylight settings etc etc I saw miniscule improvements at major sacrifices of my visual quality. In that second video I posted above we can see that this invalidation or problem is apparent even without forcing lighting information into bands in post-process.
I just don’t understand enough about Unreal Engine’s dynamic lighting system to fix this issue, and it seems to be a rare issue… for some reason.