All this guessing is wasted time and mental energy. From iPhone 7 to the latest iPhone there is not much difference worth getting excited over. They run the same iOS, business apps, log into same servers, play the same games, surf, check emails, make calls, etc. etc. etc. Purchase a new phone you have bragging rights for less than a year; whoopee.
Okay, we gain a speed bump, however iOS is the same for a two or three year old iPhone as it is new iPhone. Thus those iPhone devices are not going to have enough difference worth the cost of upgrading, essentially they are same-same.
Looking for a phone worth an upgrade, phone you purchased providing something truly new not a rehash, maybe look at one of the Android flagship phones perhaps. Owning iPhone XR using it in both business and personal my observation is most folks with iPhone use Google software not Apple (myself included). Need to ask ourselves, "Am I stressing my present iPhone, what can I not do I need to do, why is it worth $1,000-ish shaving off a few seconds, etc."? Remember, our iOS experience will be the same on the three year old iPhone as on a newly released iPhone. Myself, until my iPhone XR give ups the ghost I will not upgrade.