I was going to leave that up to my doctor because they know much more than I do. What I meant is that I’d want to check how much I need the next booster (or wait if I don’t) before actually getting it.

You're missing the point. You can take your doctor's advice, or not. But that advice will necessarily be generalized based on public health guidelines and not personalized based on any relevant quantitative tests. Your doctor doesn't have access to the assays necessary to accurately measure cellular immunity levels. Those are rather complex and typically only used in small research studies.

Antibody levels will always decline over time after infection or vaccination. A third booster might have some long-term benefits for certain patients but beyond that trying to keep those levels elevated with repeated boosters is pointless for most patients.