to be perfectly honest, i think it would be simpler to simply make a standard C language pgp-like program that has the json hash generation, BIP340 signatures/ecdh and simply make it a drop-in for all the required args so you can literally just give it the secret key in the same form as would be for ECDSA but it recognises the json, rearranges that, hashes it and then signs on that hash and then generates the wire format output