I'm considering getting a Series 10 and am in the process of amortizing its cost as part of my research.
i think Series 10 is a good value bet for the long term.
Series 9 significantly increased its CPU vs the Series 8. very noticeable difference.
Series 10's display is larger, yet slimmer than the 9 and similar to the Ultra even in display size.
one question would be about how apple is going to implement Apple Intelligence into the Watch.
assuming that almost all of apple's Watch related AI work would be done on the iPhone, then not much need to consider the impact of near term Apple Intelligence on the Watch.
lastly, since this wearable is all about putting new sensors into it for health, if health related monitoring is your primary use, then you have to consider when blood pressure trend indications might be made available (reportedly with Series 11) and then non-invasive blood sugar reading might be coming to the Watch (possibly not before 2027).
if i were to buy a Series 10, i would figure it good for minimum 4 and maximum 5 years. and would also figure into the calculation apple's new monthly apple care subscription cost as well.