Ok.
don’t take this the wrong way, but you are clearly a complete novice road rider and the only difference you’ll notice will be in your head (either from what you’ve read on here or from the perceived worth of a more expensive frame).
Having read your ramblings on here over the years it’s clear you don’t have a clue about geometry, fit, handling or even frame materials/design of mtb frames.
Stick with the PX (assuming it fits) stop buying “upgrades” for it and actually ride the thing. choose something else in a few years. ATM it’s way more capable a bike than the rider is.
£60 to alter your saddle position and tell you you’d be more comfortable with a shorter stem is a piss take.
Any good roadie bike shop would do this for a regular customer (or a new customer who’s just bought a bike) for free.