No such thing as a 5G phone that does not also do 4G LTE currently, afaik.
and deactivating 5G won't have any impact on your bill--ATT will still bill you for 5G if that's your plan and you don't use it.
But if you want to deactivate 5G, use a 4G sim card.
I think ATT contract plans require 4 lines before they offer family / multi line discount?
Switch to ATT prepaid, you won't get the same deals on hardware as they offer to contract customers, but you can get discounts for multi-line starting at 2 lines for 90$ out the door (inclusive of taxes and fees, partly b/c prepaid shit is mostly tax exempt)
https://www.att.com/att/prepaid-family/Grab a Nokia or Motorola or OnePlus Nord for a few hundred out the door to pair with your prepaid plan.
Spectrum has a crazy cheap mobile plan. I think the 29.99 spectrum unlimited plan includes all taxes and fees per month?
https://www.spectrum.com/mobilecaveats:
https://www.youtube.com/watch?v=620Ns87JtOE1. must have spectrum internet
2. must sign up as a new customer with 2+ lines
3. supposedly Spectrum's throttling after 20 GB per line is real and brutal. But throttling likely not an issue if you mostly use the device for high bandwidth uses while on wifi
Spectrum = Verizon towers
There's also Cricket, which uses ATT towers with slightly lower priority on the network.
2 lines for 40$ a line = 70 dollars a month for 2 lines out the door after 10 dollar a month auto-pay discount on cricket
I think all prepaid plans prices include all taxes and fees nowadays. Strangely, prepaid cell phone service is exempt from Telecommunications Excise Tax and possibly other wireless service taxes depending on the state/city.
It's kind of stupid at this point to be paying extra for the same service by not going prepaid.