1. Buy the cheapest premium device plus one minor upgrade (more storage etc) as that has the lowest total depreciation.
2. Get AppleCare and pay up front
3. Sell it the moment the AppleCare runs out on eBay. Don’t part exchange it.
4. Your monthly TCO is the sticker price plus AppleCare minus the sale price all divided by the months you owned it.
Make sure all the costly events are someone else’s problem.
I’m running a whole stack of apple devices with little incidental cost if anything happens and lowest overall ownership.
Edit: keep one iphone back which isn’t yet entirely obsolete if you need a backup. Still got a working 6s here.