How much do you "fix" iPadOS, before it morphs into MacOS? Fixing iPadOS is basically saying "make it more like MacOS." Don't do it half-arsed. Have iPadOS and have MacOS, the later on laptops, desktops and tablets. A third OS that sits between iOS and MacOS is not necessary.
Many, including me, have spelled out how to fix iPadOS on these forums. None of it involves turning iPadOS UI into the macOS UI.
A powerful touch first OS built on UIKit rather than AppKit would not be pointless. I actually think iPadOS and iOS should merge but that should mean bringing more capabilities to iOS rather than slimming down iPadOS.
The foundation is there for a truly flexible OS. One which can transform via stage manager on an external display with keyboard and trackpad to a multi-window power OS all the way down to a pocket phone OS.