That's a lot of rear toe-in in comparison to the front. The more the rear toes in, the less "loose" the rear is going to want to be.
You mentioned the E30...Did that have squared tires? The next logical step, is if the "understeer" is so much and so undesirable that it's making it impossible for you to get decent time in the car, then giving up rear grip may not necessarily be a BAD thing. Go 255/whatever on the rear and see if you can get better results.
LASTLY. Since you're on MOSTLY stock suspension, keep in mind that it takes smoother inputs, both hand and feet, to get the best results. You mentioned:
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My best runs were hard braking, gentle turn in, and wrestle on the edge of understeer through the turn with a hard throttle out and some oversteer through exit.
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And:
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If you asked me where I was running into it, I'd say that it was from turn in to apex.
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This would suggest to me that one of your multiple inputs up front on turn-in was too quick, resulting in the car going into an understeer state before the suspension sets. I'm going to share a simple adage that was an epiphany for me as a driver.
90% of going fast happens under braking.
Now, not having the opportunity to sit in the passenger seat, and not having video to review, whatever I have to say here is all hearsay (and wild @ss guess). But I suspect either your brake release prior to turn-in was too abrupt, causing the front end to come up too quickly and losing grip to allow you to transition into the turn smoothly, or the timing between brake release and turn-in is off leaving you in a perpetual state of understeer.
My standard disclaimer applies here though. I'm not an autocrosser. Heck my CR buddy who's NEVER AXed beat me by nearly 1 full second on a 27.5s AX course in the same identical car. So take my advice with a giant grain of salt. But I suspect, fix the braking and 90% of your understeer problems may go away. If you're still willing to take my unqualified advice, I have 2 things for you to try.
1: Brake earlier. I know it's counter intuitive, but it's the BIGGEST issue that most drivers driving heavier cars than what they're used to have a trouble grasping. And on this chassis it's imperative you don't try to be "Mr. Brembo" or play "hero" with the brakes.
2: Brake LESS. Again, counter intuitive. And this might be specific to this chassis, since it's short-ish compared to your average car, and the fact that you sit over the rear axle making the pendulum of momentum swing faster and harder, but the two combined makes it easy to overbrake and overwhelm the front end.
Of course, I'm going to tell you that if neither one of the above works for you, try braking later and brake harder.