SPICE model DC Sweep, triode region, factors other than Vto affecting knee slope (JFET or MOSFET)
Only a partial answer (as to "why can't I make the model look like the data sheet").

I grabbed a manufacturer model (ON semi, 2SK2394). This includes a simple SUBCKT which seems to just add tiny lead inductances and so forth.

The simulation result does not look "beautifully like" the DS curves(!) - Again, the model gives a "too sharp" knee.  When compared at same size &amp; aspect ratio, the LTSpice plot (fine resolution step) looks more 'piecewise' than the DS curves.

This point would  be less obvious for another part with a more negative Vto though.

I've a feeling that - for small Vto parts - I might struggle to match both saturation &amp; triode regions with the same model.