use iPhone 12 for camera only?

You can use an iPhone that does not have cell phone service for everything except cell phone calls, text messaging, and cellular data access. I used my iPhone like that while I was waiting for my carrier to mail me a SIM card.

This includes taking photos and synchronizing them with a computer using iCloud Photos. (Assuming you do have Wi-Fi Internet service, and sufficient space in iCloud.)

If you had a contract with Verizon, especially one that let you pay for the phone over time, or that subsidized the price of the phone, there might be costs associated with dropping cell phone service too early. You would need to check with Verizon about that.

I kinda' do that with my retired moto e Android phone. Essentially it's like using an Pod touch, except that moving files to/from my MacBook Pro is a rather primitive process.

If you want to cut costs for cell service, check your provider for some sort of "Seniors" package, which may only have text & talk, no data. We've managed to hang onto that sort of pricing plan for my whole family for years now, starting with Doro flip-phones and not bothering to "upgrade" to a data plan when any of us got smart phones (3 out of 4 so far).

For transferring photos to your laptop, just use wi-fi or (better yet) plug it in with a USB cable and sync them with the Finder (or iTunes, if Windows). You can still use iCloud storage too.

Do the same to move music, e-books, etc. to your iPhone.

Siri still works, with a wi-fi connection. Even without wi-fi, Siri can do some things like carry out Shortcuts, set timers, and so on.