If the high poly model fixes it, then it likely proves that it was the shadow terminator problem. You should go back to a lower poly model (maybe not as low as it was before though) and try again to get a good bias.
Standard shadow maps have shadow terminator problems too, although they are less obvious due to being blurrier. You shouldn’t have tech issues with CSM, they’ll just look worse.
Keep in mind that while it may have 100k tris, Nanite is basically never going to actually show 100k tris - so I wouldnt think too much about the max.
Solving the terminator problem is about finding a healthy balance between tris and bias. Too much bias and shadows won’t look aligned. Too high tris will be a waste of memory even if Nanite can render it okay.