there’s basically no way you can find no difference between the trackpads on the Steam Controller, and the trackpad on the PS controllers

Never said there’s no difference, just that it’s good enough to navigate the desktop on a couch.

Does a PS 4/5 allow you to fully remap all the controls of a game to literally whatever you want?

Not natively, but they’re a normal directinput device so there’s plenty of software out there to do it. Plus steam input for steam games.