HbA1c (glycated hemoglobin) is both something that matters and something that can be somewhat easily measured (it's in the blood so no biopsy required. AKA 'liquid biopsy'). It matters because it's systemic, has effects all over the body, but particularly the periphery - and it's indicative of how well a patient has been managing glucose (dependent on how quickly RBCs recycle, a per-individual variable).

Glucose in excess can indeed bond any (and every) where in the body. an observable one is glycolytic damage to the lens in the eye (a place not visited by RBCs).

There's also potential damage from high insulin, primarily in accelerating cancer - and it could have a role in AD.

BTW, I'm not a Dr, just got interested a couple of decades back...