So I was curious how bad the options are at the 23" size, and it's bad. You have exactly 2 all-season options at the 285/40/R23 size, but no matching option at the rear as you've seen. Then, you have 0 all season options at all at the 325/35/R23 size - if you go to a 305/40/R23, you do get 1 all-season option, but this tire doesn't have a matching 285/40/R23 tire. That's all a long winded way to say you're kinda screwed whether you go all-season or winter (which there are 0 options as well).
I can't imagine the GLS600 couldn't fit 22" wheels, which would exponentially expand your tire options, e.g. 325/35/R23 -> 325/40/R22 and 285/40/R23 -> 285/45/R22.
Tire results for going to a 22":
https://www.discounttire.com/fitment...rear/325-40-22