I agree that metrics should be tied to actual individual income and not the stock market or GDP. (As an aside, my cynical belief is that we've always known those to be poor metrics but we use them anyway because they measure what people with power actually care about). But keep in mind that even median income is not a good indicator of economic standing with high cost of living. Whatever it is would ideally capture how much spending/saving power is left over after essentials (warmth, shelter, food, water, health etc).

That’s why I mentioned something like purchasing power.

In the end what really counts is the health of members of society as a whole.

Providing everyone with basic life necessities would be a good start.