VC4 500mm IDEX

So I've been at this for quite a while now. I've made sure the gantry is square to the back motor mounts with no belts on it. I then added the belts on. I kept Y loose (almost slack), tuned the X and DC belts without issue to get either MZV or ZV recommended with usually 0% vibration. Both T0 and T1 X look good, and are giving what I think are reasonable accelerations with MZV (upper 6k) - first photo is representative). I can also usually get T0 Y to be pretty decent, all things considered. T1 Y has been a significant problem, often with an unknown peak around 90hz. I occasionally get one around 125hz, however cannot find anything loose or vibrating on the toolhead (and I have T1 wire managed the exact same way as T0, which does not have this issue). Furthermore, while I can ultimately get somewhat decent graphs and accelerations at the end of the day, I get significant dynamic skewing during printing. I suspect this is because my Y belts need to be so loose to achieve good Y IS graphs. Tightening them even slightly produces horrible Y graphs (third photo). I've heard that the sweet spot for the Y belts is small, but I've tried going by 1/8 or even smaller adjustments, and still end up with worsening Y IS with practically any tension on the Y belts. I've rechecked that the gantry is square even after all the belt tensioning. I've rebuilt both toolheads. My X and DC belts are identical lengths, and have the same number of teeth sticking out of the rear belt grabber on the toolheads, making them an equal length. I've taken off the Y belts and verified they are the exact same length, and placed in the holders to the exact same tooth. I am out of ideas on what to do. I cannot find any suggestions from searching, other than the usual that the belts need to be looser than most would think. Unfortunately, it seems my belts have to be so loose that the gantry skews and prints crooked walls if I use any acceleration above 5K, which doesn't seem correct.