The brake lever on my bike is not adjustable. The brake switch on my bike is not adjustable. I've never seen a switch mechanism like that on a motorcycle rear brake before, but there you go. It's entirely possible my GS was the same, but as I never used the rear brake on that in normal riding (the linked brakes operated by the front brake lever worked absolutely fine, so I only used the rear brake lever when doing slow speed work) I never had anyone tell me the brake light was coming on early and so I never noticed.
I've changed where I position my right foot by angling it to the right and solved my issue that way. I've been doing it for a while now and it's fine.
If those people who say it's easy to adjust the operation of the switch would be kind enough to post a photo or a diagram showing how they did it, I'd appreciate it. 👍