FWIW I’m in the same boat.
For steam I have been settings my game launch options to
prime-run %command% to trigger dGPU to be used. %command% will be autofilled by Steam.
For other programs (ex Unity Editor) I have to find how Unity Hub launches the Editor and then prime-run the Editor directly.
It’s not great but it has worked for me so far. My main issue is external display / TV related or the issue with Powermizer not respecting performance modes when you unplug form AC and replug back in. It’s not exclusive issue to Manjaro though, I confirmed it happens on Pop and Mint the same.
You can confirm you have the same issue if you open nvidia x server settings (GUI app) and check powermizer. Set to maximum performance while plugged in. Then unplug and replug back in. It will never be able to hit the max power mode anymore until you log out. More on this here: Set Nvidia GPU Performance Level possible? - #4 by Yochanan
As for wanting to have it completely deactivated until requested. IIRC that is not possible without switching to bumblebee. Prime will always have it activated and at the lowest power state (3W ~ 6W). Otherwise if you lack the driver it will always be at way higher (25W or so!) since dGPU will init at system launch.