Peter Attia & Iñigo Sam Milan had a pretty good dialog about Z2 cycling a little while ago.
In that conversation they say that the point of Z2 is that you’re training below your LT1 threshold and Lactic acid is not accumulating. In absence of a lab and blood work to test for this during the ride, they give as a rule of thumb checking that you’re able to hold a conversation while riding.
This points to HR and breath being a more effective metric to ensure that one remains in Z2 and power being something that one checks after the ride.