Thats the problem when dealing with high mileage cars, what repairs are worth doing? I had my 97 Explorer in the shop (146,000 miles) for grinding brakes and it came back $2600, new 4WD hubs, all new brake pads and rotors, a bad axle seal, bad ABS sensors, all kind of things after spending $1500 in Early December to repair the gauge cluster and do routine maintenance and flushes. I know how you feel. I figured hell, I've got so much invested in it now that theres no point in selling it until something major breaks down again. The $1590 was mostly elective work like with you.
Fact of the matter is that the Lexus is a reliable car, moreso than my Explorer (there hasn't been a service under $500 in like 3 years lol) and probably at this juncture I would say its worthwhile to spend some money on it. Depending on the transmission replacing the torque converter is usually a partial rebuild at least. Since you haven't frozen any gears yet and the tranny still shifts I would take it to somewhere like AAMCO and see what they think. On a car as old as that one is, even with your mom's discount there's really no reason to service it at the dealer. In all honesty the dealer is so expensive theres no reason to service a brand new one there...
Keep us posted ;)