First, sry for my bad english.
Thanks for going through all the trouble to find a solution.
I bought BL3 at launch and played on DX12 ever since. So you can imagine i was quite annoyed from the “not optimizing problem”. Because you could not work out the problem I had the Idea to search for command line arguments and one particular Website came to my mind (Borderlands 3 - PCGamingWiki PCGW - bugs, fixes, crashes, mods, guides and improvements for every PC game)
Stuttering on high-end hardware • Link
There have been multiple reports of the game stuttering on hardware exceeding the recommended system requirements.
Switching to DirectX 12 may result in better performance and reduction of stuttering, as well as better utilization of the CPU in combat on high-end hardware.
There you can find an argument (-NOTEXTURESTREAMING) which changes everything back to normal so there you go.
Till this day i can not understand why support didn’t mention this small one liner and instead lets you go through such trouble.
I hope that I could help at least someone