re: Data only plan on old iPhone

Mint and T-Mobile both sell SIMs designed for tablets. All of the majors seem to, but just go to your preferred carrier and look for tablets or devices.

ETA: Caveat, I don't know how old the phone is, but make sure to check if it accepts e-SIMs or not, that is going to limit your options if you can't use an e-SIM.

ETA2: I know the Mint page is for "phones" but they're an example how how you can even get voice you're not going to use. Voice and SMS are so low value to carriers these days it's kind of a throw away (except for international.)