I will usually buy a flagship phone with the expectation that I will need a new one in about 6-7 years at least (which the iPhones have been really good for so far, the one I have semi-retired is the XR which I think was in its final run before being superseded by whatever came next when I got it). I don't know how long my current one is going to go for, it is modular but it got discontinued last month (of course it did -_-) but parts are apparently going to be available for another two years and I am pretty sure the batteries they use are the same as for another of their phones which is still in production.

Good luck picking a phone XD