Buy mosfets with 4 terminals
Separation, of Substrate/bulk/well from Source, is a standard method to greatly increase the threshold of a FET, and let the differential_pair respond linearly to input signals above or below the rails.
You can of course use this method with a single FET, but I've only used it for diffpairs, in custom onchip &quot;opamps&quot;.
Or in very fast cheapo level_shifters for differential signals.