I know you're ready to throw in the towel, but one thing before you do, concerning your third statement, don't turn the polycount down, that's what was causing my balding problem, what you want to do is export the highest poly mesh, and import it to the other LODs with the lowest poly (resulting in each LOD having the same polygon/vertices count); lowering the poly just turns the quality of hair down (I guess this could help if your computer struggles with quality, but if not, it's often the game assuming you are, and forcing you to see bad quality while it load). Honestly, I don't know if this will help you, the hairs could just be faulty, but the LOD is what tells the sims to show during the process of rendering quality, and if each LOD is the same, then it'll force the game to go into the highest quality on start up at the speed of your graphic card's allowance.
Ok, after 1 day of "fight" I can post some updates from my problem...
1) Settings are full, resolution is 1920x1080, texture uncompressed and, of course, no laptop mode
2) I can play AC Odyssey without FPS loss so I assume my graphic card is ok... I don't think Sims4 would be more "resource hungry" than Odyssey
3) I downloaded Sims4 Studio, Blender and followed a tutorial that shows how to reduce polycounts and.... nothing happened
I'm going to explain... I noticed that many hair have lower polycounts as Lod increase. So, starting from LOD0 (the highest polys), LOD3 very few polys. So I exported mesh, used the "decimate" option in Blender and reduced of 50% polys... Nope, nothing happened, hair look awful in game (of course) but always still need 1-2 seconds to load. So it's not a matter of polys... I'm about to drop everything and accept it... just wanted to let you know my personal "odyssey" eheheh
Take care. Thank you again...
Sorry nothing is working for you, I can't imagine what it is. Well wishes to you too, have fun simming!