There isn’t a aftermarket steel pan for the 10r80 yet as far as I’m aware, there are aluminum diecast pans with drain bolt but they are too expensive IMO. As for $600 for a drop the pan fluid change, I think it’s reasonable, I just did it myself, the oem filter along is around $100 plus another $100+ for 3 jugs of ULV fluid, that’s $200+ in parts along. I reused the stock pan & gasket, sucking out the fluid from dipstick hole isn’t too bad at all. The hardest part for me was to wriggle out the heat shield after taking out the stud bolts so to gain access to the rest of regular bolts.
as for manufacture recommend 160,000 mile change interval, I personally don’t believe that… did mine at 50,000km, one quarter of my miles are towing and a it seems easy enough I’ll do it often.