we have many topics in forum about “limited by mainthread” and also the currenty DX12 impl not fix that at all. ( reason: all is waiting for that one ‘worker’ which can not splitted more to other cores, therefore you see one core in full load ). The most common thing to reduce that effect is: reduce TLOD setting e.g. to 150 and not go higher than 200.
EDIT:
and I forgot… your GPU is at 81° … that never normal, if you not own a laptop, with 12% load.Thus try to change the “copy” engine to “graphics_1” engine… See also Answer : Why is my GPU usage only 5..15..25.. %
PS: mentioned the EDIT, because that is what to upgrade 
… I assume your 2060 is in full load, independend that you currently in some situations cpu limited too.