you can polish clearcoat it will still get microfine scratches
notice the swirl marks in the clear coat on the left of the photo. right side has been polished using megs 85
if you look on detailing sites then wax / sealant is known as LSP - Last stage protections.
LSP adds very little to the actual finish of the car. its to protect the paint. the problem is understanding the difference and that some polishes actually have sealants in them. AG SRP has a sealant in it
my car after being polished last year
different angle but after 2 coats of sealant
the shine comes from the paintwork itself, not the LSP, if the paint is perfectly smooth then youll get great shine, any scratches or dirt then they will distort the reflection - hense why your car looks better after a clay session.
i strongly recommend claying before polishing, especially machine polishing, as the dirt trapped on the paint will cause more scratches.
HDi if your looking at a detail then look at a good couple of hundred depending on how the paintwork is