I found OEM black 5 spoke wheels for my GTS. The wheels are 19x9 ET62mm and rear are 20x11ET68mm.

My dealer recommended 305/30R20XL for rear and 265/35R19 for front. The original tires are 295/30R20 in the rear. Does this sound right? Will the 305’s be too big?

This is is an expensive proposition. Almost 10k!!

not sure, it’s going to be worth it

on original wheels. I’m running 255/35/19 size tires up front and 295/30/20 without issue. Michelin PA4 Alpin. The whole setup cost me about 2k

i was looking to get rims and tires so I could switch out the wheels myself as needed

You could save many thousands by purchasing custom, but OEM sized, wheels by a company such as Signature. The OEM AMG wheels are a great wheel but so overpriced imho.

https://www.signaturewheel.com/