It’s not just config though (unless you consider logic to be config). When you’re roaming, the sim applet has to generate a path back to its home network based on request/responses with the networks it can see and their partners (and their partners’ partners etc.)

It’s effectively multi-hop peer discovery and I don’t think you can encode the general case logic for it as just config.

Edit: as a (rather niche) example, FirstNet sims run a different applet to AT&T sims despite nominal running on the same network because they have special logic to use more networks if they are in an emergency area.