I tend to buy the best phone I can (e.g. iPhone 15 pro max) , and keep it as long as I can. I think this maximizes the enjoyment and value I get from the phone hardware. Usually I'll go for 0% financing under contract, and when the contract expires switch to a cheap prepaid plan. I've found the difference in service between postpaid and prepaid negligible, provided you go for the right carrier and plan (I highly recommend T-Mobile connect, and AT&T annual plans). Usually when buying a new phone trade-in credits and promos make it cheaper to sign a contract than to pay cash for the phone, otherwise I see little reason to go for postpaid unless one relies on mobile data for their profession (maybe 1% of people need more than 10-15GB per month per line?).
Other people I know buy middle of the road phones (e.g. iPhone 15) with cash every 3 or so years and use prepaid plans, which over time comes out to be minimal savings over using flagship phones, especially with 0% financing. Personally I find it much more enjoyable to use a flagship phone for 5 years than get a middling upgrade every few years. Lastly, there's those who buy a 5 year old used phone every year and sell their old one, which is clearly the way from a purely financial standpoint, but I personally don't see the harm in spending on something I interact with every day.
*My* Strategy, that works for *me* is to buy (paid in full) phones that are 1(ish) model behind the newest and greatest. I look for good deals, including reviewing refurbished deals from reputable vendors, always with liberal return policies. I then find pre-paid service plans that provide quality service for good value in comparison to subcription plans, -always with a major carrier network.
The above is a winning hand for *me*.
HOWEVER, what I don't have in the above scenerio is the phone insurance offered by a couple of my credit cards, as those cards seemingly require that you pay the *monthly* subscription bill with the card. I don't have monthly subscription, I buy a year's worth in advance to maximize ROI. I doubt there is a remedy, perhaps others have wisdom.
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I buy my phones outright because I'm grandfathered into a legacy Verizon plan. They can't throttle my unlimited data. I've chosen to not upgrade my plan because the rules for unlimited data constantly change every time they modify their plans.
My wife and I are grandfathered into a Metro by T-Mobile plan which was 2 lines for $50 out the door per month. Since then, we accepted an offer for a free third line if we did a phone upgrade. So, now we have 3 lines for $50 / mo out the door. Pay with AOD so it is really $48.50, lol. The extra line came in handy when our 13 year old requested his first phone for his 13th birthday.
It technically is/was a 2GB data / line / mo plan. But no extra cost if you go over. They just "may" move us down to 3G. Well, at the time (2017) Metro/T-Mo still had a 3G network, now they do not. It's 5G and as far as I know, nothing happens anymore if we go over 2GBs in a month.
Our first Android phones were free and, since 2017, we may have done ~3 phone upgrades each, paying only the upgrade price and never more than $100 - $150 for a phone upgrade as far as I remember. Right now I have a Moto g stylus 5G and have zero issues with it. 
Personally, I couldn't fathom paying $1000 for a phone. That's just me!
