No matter how long you want to keep it, I recommend you the new SE, I have one and the supposedly big differences to the 8 are mostly marketing. According to Apple the SE has a much smaller display by marketing the 8 as having "nearly 20% more.." screen real estate which of course means the difference is less than 20% and whether it's 16% or 19% - while the numbers sound big it just doesn't make a difference when you check the time or get a notification. One of those things you only notice when you put both on your wrist next to each other.
It does not have fast charging but from a low charge state it merely takes an hour to get up to the 80% realm. I place it on the charger when I warm up lunch and later dinner, that's 30 minutes each. Not having the AOD is also beneficial for the battery runtime, with AOD the runtime is noticeably shorter. So I don't consider it bad that the AOD is missing from the SE.
Otherwise it is mainly missing sensors, it gives you irregular heartbeat warnings but the ECG app is not available, the blood oxygen app isn't either and no temperature sensor for ovulation tracking. The heartbeat is checked with the optical sensor that is also used to get the pulse. So it doesn't have the ECG hardware - but I never once used ECG since it's a whole thing... to get a reliable reading you gotta sit down and relax, and then touch the side with a finger of the other hand for 30 whole seconds.
Otherwise you'll get an "inconclusive" message. And even if every full moon you actually want to get an ECG reading the most the watch will tell you is that it's a normal rhythm. I mean, the 8 costs more than 50% extra over the SE price, so what tangible benefit does this feature have? The sensor is apparently very accurate, but again, in daily use who does regular ECG readings and what are they good for? If you already have a medical history in that regard, it could be different. (Similar single lead medical grade ECG devices cost in the quadruple digits.)
There is no stainless steel version of the SE, and it is not IP6X certified to resist dust. Same water resistance rating though.
I really don't see what I pay 60% extra for with the 8 - in different words for the price of one 8 I can get the SE 1.6 times. Unless you really need a feature that is excluded from the SE the 8 is way too expensive.
