Switching to T-mobile from AT&T via Costco. Confused at all the promotions

Hi all. I’m fed up with att and ready to switch. I’m a bit confused at what they offer to switch. Currently have an iPhone 17 pro and iPhone Air. Looking to upgrade the 17pro to the Duo in a couple weeks. I’ll get up to $800 to pay off each devices from att. It looks I can upgrade to the Duo with no trade in if I’m on the Beyond 2.0 plan, right? It doesn’t look like the promotion for the Duo is out yet though, so I assume I’ll pay a small amount over 36 months. But I can then sell the 17 pro since it will be unlocked? Or would it make more sense to go on the lower plan and trade in my iPhone 17 to T-Mobile? Costco is also offering an additional $150 Costco Digital Shop Card + $250 Prepaid Visa Card. Not looking to upgrade the air yet. Does this sound right?