The "standard" for rear brakes is the ability to lock the rear wheel and cause a skid. They should do this easily and reliably.
My instinct is to suspect surface contamination of either the pucks or rotor, but you seem to have eliminated that.
Reaching blindly for straws, have you checked the cable? It may be kinked, frayed, or have a damaged housing.
Another possibility is something affecting travel of the actuating arm causing it to bottom out internally rather than against the rotor
Last edited by FBinNY; 09-18-22 at 10:15 PM.