Dealer diagnosed slow leaks on both rear tires as cracked wheels(!) and
quoted replacements at $600 EACH ! Seems to me, I could buy 4 decent
new wheels for $1200 and sell the front wheels to offset my costs.

Rear tires are 275/35 R 18, but I don't know their width/offset
(fronts are 245/40 R 18)

Are "matching" OEM wheels available aftermarket? Any suggested sources?
(assume the hub roundel pops out and can be transferred)

Any mfg suggestions for a full set of wheels of the original quality?
(Considering the weight/power of the car)

Any reason to explore "repairing" the cracked wheels?