Not sure why I’m mentioned.
IINM you can redeem an insider code within 30 days, but I’ve never done it personally. You’d have to get the code though, might be easiest to get one from a Costco T-mobile rep.
Go5G Next is only worth it if you want a new phone every year, otherwise the premium you’re paying for the plan isn’t worth it. In reality you’re subject to whatever phone promos they’re offering when the year is up.
Gotta consider that resale value for a one year old is less than purchase price.