I tried the Q5 before getting the GLC, I also have galloping arthritis and spodylosis (curvature of spine) The Q5 was shocking! the seats were wide but very hard, they hurt within the hours test drive.
I have the GLC 250 AMG with premium pack, not all singing all dancing.
The only thing I did was have the black alacantra leather replaced with soft grey leather from a specialist. The difference was amazing both in comfort and looks. The only problem I have is the "vehicle too close warning" it really annoys me and goes off at the weirdest of times.
I hope you get your problems sorted.
Maybe look at the Lexus?