> But once you buy a car, the metric completely decouples, and no longer points towards your original fitness goals even a tiny bit.

True, but if what you care about is "how quickly and safely can I reach a given goal", distance traveled over time is a great initial indicator, and accident rate will help illuminate.

The question "does AI help me move faster towards a goal, at the same quality standard", is relatively easy to judge in a solo project. As long as you verify equivalent standards, and don't play in an area you don't know at least - folks have a pretty clear understanding of their own productivity if it's a familiar thing.