The real problem is that you got that SE 1st generation at the end of 2018, but that device was firstly released on March 2016. That's why I think that your device won't receive new operating systems like iOS 15 or 16 (generally released during fall season of every year).
But, since your iPhone SE is still supporting iOS 14, you are okay untill 2021 fall. After that date, it doesn't mean that you cannot use that device. It will keep on working, but perhaps without iOS updates.
And you can subscribe a contract with your carrier, asking them what happens if you decide to change your device before the contract ends (I believe that, if you decide to stay with that same carrier, nothing will happen).
Regards
Giulio