PN is all about the edge rate and so is amplitude, if you
are "gain starved" - may need more than one stage to
get squared rail-rail and fed-back single stage inverters
don't have the greatest voltage gain (low Rout, sub-peak
gm). It could be that you'd be better off with one more
common-source gain stage and a capacitor-blocked
connection to the ->digital buffer chain (you will impose
less loading, one gate vs two, and perhaps enjoy a better
Rout (one drain and one load, separately optimizable, vs
a pair of low-Z drains).