The best option would be a software-only eSIM with full transfer support, IMO. But we don't have that, because GSMA says we can't have nice things.
Yep, I remember a time where you could extract the Ki and IMSI from legit SIMs and write that to a bog standard Goldwafer card (which were also used for cable TV hacking) including some SIM emulation software and thereby clone the SIM. That was like 30 years ago and the only thing that changed in SIMs since then is better encryption.
The obvious issue being: it's pretty hard to acquire raw key material. Most vendors refuse to sell it, and the workarounds are messy.