The upgrade dilemma - iPhone 18 ProMax vs S26+

I'm on the Experience More w/55+ plan (auto migrated from Magenta Max 55+ in July) and I'm currently using an iPhone 16 Plus. I was excited for the 18 ProMax until I saw what the upgrade offers were for this plan type. The most I can get in trade is $730 for a $1300 phone. Not ideal. However, I also have my old iPhone 13 which is in very good condition and TMo currently as an offer on the S26+ where I could trade in the iP13 and get the S26+ for free (plus tax, of course). I used to be an Android person but haven't used one as my daily driver for 8 years or so. I'm used to the iPhone ecosystem. But it seems like the better deal is to go with the S26+. I should also mention that there is no reasonable trade in offer for the iPhone 13 when applying it towards the iPhone 18PM. So that's not even in consideration. The choices are, iPhone 18PM trading in the iPhone 16 for $730 off a $1300 phone or the S26+ trading in the iPhone 13 and getting the Sammy for free (24 months of credits). I feel like this is the last chance to trade in the iP13 for full value and this allows me to keep the iP16+ as a backup device. Someone please point out the flaw in my logic.