I ran this analysis last year when I got my wife a 14 Pro and the cheapest overall option (considering total cost over a 24 month period including the phone, service cost, and any rebates or credits) was Mint Mobile (operates on T-Mobile network). Like around $900 cheaper over the 2 years.
Most of the "deals" are the carriers getting you to sign up for their expensive monthly plans, generally not worth it unless you have extreme mobile data usage, a ton of phone lines (maybe), or some combo of those scenarios.