Thanks to fadingsignal for having his mod with the source available so I could see what base game function to call to hide the hud.
Donation Points system
This mod is opted-in to receive Donation Points
Uses the same papyrus code fadingsignal used but with an hotkey in an MCM to hide the UI.
I made this cause a friend still uses fadingsignal's mod but doesn't like the potion in the inventory. and all the other quirks it has due to being one of the very early mods.
Install MCM Helper (Required) Install the mod with a mod manager.