If I trade in my phone, I want my trade-in offer now. Instead, the full value of whatever trade-in deal provided is split over 36 months as bill credits.

While the new phone might actually be “free” in one of these promotions, it’s not, naturally, because you’ve been thrown into a 36 month installment agreement separate of the cellphone service they’ve sold you on (that they also claim is “price locked” while independently raising surcharges and other fees).

Then I'd just buy the phone outright and sell it at the end or use another trade in route through Apple directly

Sell your phone, never trade it in. That's the surest way to lose money.

Do a little more work and sell it on Facebook Marketplace.

Convenience comes at a cost.