You have a few other options as well when buying the phone through Apple:
iPhone Payments - this is a 24 month, 0% APR financing agreement through Citizens One bank. This is just like the iPhone Upgrade Program, only without the 12 month upgrade eligibility, and AppleCare+ is not included in the monthly cost (but can be purchased separately as either a monthly or annual subscription).
Apple Card Monthly Installments - sounds like you’re already aware of this option. It’s also 24 months at 0% APR, and you also get 3% cash back just like any other transactions at Apple. If you already had the card, then I’d say this would be a better option since you just have to apply for the card itself, and don’t have to apply separately for the installments. If you don’t already have one, it still may not hurt to apply, as it will not run a credit check unless you are approved and accept the card.
AT&T Financing - this is a 36 month 0% APR offer, and would be billed through AT&T. However I know you don’t want to be committed to them, and this would mean that the phone is locked to AT&T until paid off. However, if you have an eligible unlimited 5G data plan, and were also planning to trade in your old iPhone (assuming it’s within a few years old), then they have a great promotion which would give you up to $1000 in trade-in credit in the form of credits on your monthly bill over the duration of the financing term.
Also note that all of these options will require that the new phone be activated with AT&T, but only the AT&T financing option would mean the phone is locked to them. Otherwise, your phone will be unlocked even if it’s initially activated with AT&T.