A PinePhone would work fine for that I think. It's also possible to use any MSM8916 phone together with postmarketOS to do that kind of thing, the G4 is not sup

A PinePhone would work fine for that I think. It's also possible to use any MSM8916 phone together with postmarketOS to do that kind of thing, the G4 is not supported on mainline in postmarketOS yet sadly, only the G4 Play. I would recommend to use usb tethering though instead of wifi tethering. Just get a router that has an USB port that supports a modem and use that with a phone.