I just turn on when I arrive in the US and set to Primary for Data & turn off data switching. (Not that I need to worry as I’m in prepaid)
I leave Voice as my Public Mobile SIM so I can so receive text messages & iMessage still works with my phone number. (Once again, I’m prepaid, so I don’t get dinged anything / if I need Voice, I add a US Voice plan thru PM for $8)