Zndl points out correctly that the trick is to not update the watch past watchOS 8. I would hope the watch series 7 would still ship with watchOS 8.

The most recent watch that maxes out at watchOS 8 is the series 3 but that one is pretty underpowered, so I’d have a hard time recommending it.

Looks like you’re right. For the fine print to be right, Apple Watch Series 7 would have to ship with watchOS 8. Do you think they’re still shipping like that? What a weird dilemma.

It looks like anything from Series 7 down will work with your wife's SE with iOS 15.

The small print from the Apple Watch Series 7:

Apple Watch Series 7 requires an iPhone 6s or later with iOS 15 or later.

iPhone SE was released after the iPhone 6s and can take iOS 15 so you should be good