NMOS is the current-error-free cascode. Closing the loop
on Rx voltage enforces a current that has nowhere to go
but out the NMOS drain (diode leakages aside, which on
a crazy big FET at high temp is not to be neglected).
Operating saturated. your op amp has a lot less work to
do in rejecting power supply noise (a better source
follower). If you have the headroom then subthreshold
probably doesn't buy you anything (and to operate in
subthreshold at such a current means an even bigger,
leakier-at-high-temp FET).