is iPhone 6SE the right choice to pair with a Series 5 Apple Watch?

There's no such thing as a 6SE. The new SE looks much like an iPhone 6 or 7, but that's where the similarity greatly ends. The internal hardware of an SE is far ahead of a 6.

There is no iPhone 6SE.

There is an iPhone 6S. It was introduced in 2015 and discontinued in 2018.

There are two iPhone SE models; the original released in 2016 and discontinued in 2018 and the current one released in April 2020.

The current iPhone SE is very popular and very well reviewed .