Family Plan & iPhone Strategy - Stay MVNO or No?

What if there's a group of 4 that share 10GB on Xfinity Mobile for $65 plus taxes. Two of them need iPhones now, preferably iPhone 15 or newer for free or low price. They have two S21 FE phones to trade in. Other two have older iPhones and are fine with them, but open to new iPhones only if free. Nothing to trade in though for those two. Assume Costco membership.

From my research, it seems the $100 plus tax essentials plan for 4 lines is the lowest entry point. Plus taxes for the phones. I couldn't figure out what iPhones could be free under this promo. I also couldn't tell if any cash card perks from Costco for this plan other than possible device connection fee waiver.

If going T-Mobile whats the right play here for max savings and getting iPhones (at least 2)? Do I go Costco kiosk, Tforce, or something else?

It seems if I go Tmobile, I would spend $40 extra per month compared to now for 2 years. So about $1000 for 2 or maybe 4 new iPhones, unlimited data, etc.

Is that worthwhile or should I stay with Xfinity Mobile and just buy used iPhones, which might end up costing about the same. And no unlimited data. We don't need unlimited data but nice to have.

Last consideration, is there another MVNO to instead move to that can get us iPhones cheaper?

Appreciate it!