Question about using Montgomery form for elliptic curve operations on bls12-381
I figured this out, the best approach is to use either Montgomery or Barrets algorithm for modulo reduction, Barrets requires a slightly higher bit multiplication but not pre-transformation, while Montgomery has a pre-transformation but multiplications are slightly faster.

And if you use Montgomery you can do the entire point double/add in Montgomery form using jacobian coordinates and just convert back at the end.