The first sentence here is in opposition to the final sentence. If you want to measure ‘general wellbeing’, you will have to define some specific metrics. These will then - according to your own argument - become the goal and you will again be in the position of missing the thing you ‘actually’ care about.

Alternatively - you could pick a good general measure (like GDP) and also measure other things alongside that to get a good holistic picture. Which is what we already do.