SOLVED.
I’m honestly gobsmacked.
Huge shoutout to Framework Support. They helped me narrow this down in just a few days (it would have been even faster if I had replied more promptly on my side).
Since Safe Mode completely eliminated the touchscreen lag, I started isolating third-party services through a “clean boot process”, so to speak.
Long story short: the culprit was Framework Control
I had installed it shortly after setting up my FW12 to tweak fan behavior.
For reasons I can’t fully explain, it 100% interferes with the touchscreen sampling. With the service enabled, I would get intermittent stroke drops and straight-line artifacts while writing with both stylus and finger. In Safe Mode (or with the service disabled) the touchscreen works flawlessly.
My guess is that something in its power management or hardware polling routine conflicts with the ILITEK touch controller / I2C stack, but I don’t know for sure.
Yes, the FW12 fan can be a bit “enthusiastic” at times, but not nearly enough to justify broken touch input.
I hadn’t used Windows in over a decade and switched back specifically to buy this Framework laptop - which ROCKS, by the way. Seeing the stylus finally behave properly after chasing this issue for days was incredibly satisfying. I was so pissed, it was unusable before.
Hopefully this helps someone else avoid the same rabbit hole.