Pair an Apple Watch (SE) with Cellular to a iPhone that only uses WiFi (no cellular - no plan)

It won't work like that.

The watch replicates the paired iPhone's service and uses it for calls and messages. It does not work on its own.

If you set up the watch using Family Setup, then it can use the cellular service on its own, but it needs to be setup with a functional iPhones and will of course lose some features when in Family Setup. You cannot have it both ways no.

In short, no there is no way to use a cellular Apple Watch without a paired iPhone and be able to use all its features. They are not meant for independent usage like that.

I use an Apple watch 6 with an iphone xr that doesn't have a sim card and everything works perfectly,the only occasion I need to use wifi is when I update the watch.

You cannot make calls or send texts but if you're fine with that everything else works just fine.

Then you will need a sim card in the phone.