My Series 2 was bought in January 2017 and would have been useful until about September 2019, when I did upgrade to a Series 5. At that point it was clear that the Series 2, though it still worked, was extremely slow and almost frustrating to use. I just upgraded in October 2022 to the Ultra, but the Series 5 remains more than fine, and I’d say the same about the Series 4 (introduced in 2018).
The hardware processor improvements since the Series 4 have not been all that dramatic.
The biggest issue? After three years, the maximum capacity of the battery in my Series 5 is 87%. So I’d say about four full years is probably a best guess of useful battery life, barring, say, a dramatic improvement in battery technology or performance that means Apple stops providing watchOS updates to watches as old as they have historically provided them. (The Series 3 received updates for 5 years, but the last year it was clear that the S3 was just not meant to be used with watchOS 8 - updates for many people meant unpairing the watch, repairing without restore, running the update, and recreating watch faces, etc.)
As for the bands, on my Series 5 I wore a sport band almost exclusively. (The same with the S2, and I still have my old S2 bands, but I never wear them with the Ultra.) It remains just fine. I do wear the sport loop during the months of July and August when I am in the water a lot, and they are also fine. I also have a braided solo loop - it’s a great band, but pricey, and it tends to stretch out over time. And the solo loop, the rubbery band without adjustment of a similar material to the sport band - I have read from lots of people on Reddit has a tendency also to stretch and to tear at the end of the band. I’d recommend the sport band over the solo loop for that reason.
I like the functionality of the sport loop most, but I don’t love the look of some of the color combos that Apple has made the last few years, until this year - I think the current group of choices of the sport loop look really nice.
For me, always on display is a necessity, so I’d not get an SE myself, but if it’s not so important to you, and you don’t have need for the ECG app (I really don’t myself), then the SE2 is a great watch.