Lexus SC tire pressure ABS and steering issues

The symptoms you describe (ABS lamp, steering wheel vibration, increased steering effort) suggest more than just low tire pressure. While a defective tire valve could cause pressure loss triggering the warning light, these additional symptoms could point to issues with the ABS system interacting with the steering. Have you checked the tire pressure manually? Also, does the shaking increase with speed?

Checked the pressure manually, and they were low, but not dangerously low...and yeah, the shaking gets worse the faster I go. Could a faulty tire sensor really mess with the ABS like that? Is that even possible?

A faulty tire sensor is unlikely to directly cause ABS or power steering issues. However, significant pressure loss in a tire will affect handling and stability, and that could indirectly trigger the ABS if the system detects uneven wheel speeds. Given the severity of the vibration and the power steering issue, and with the mileage at 194481 km, I would advise having a workshop inspect the car. It is probably better to have it checked professionally, as it could be more dangerous than you think.

You were absolutely right! Took it to the shop. Turns out a valve in one of the tires was defective and losing pressure. They replaced it, re-balanced the tires, and the ABS light and steering issues are gone! Cost me 280€, but well worth it for peace of mind. Thanks a bunch for the help!