The idea's good. Have a performance part that's turned off most of the time to save power. ARM's little-big works OK. Optimus, though, is having two completely different graphics stacks that can't actually communicate with each other, and the high performance part can't actually be turned off. Nvidia's Linux support of Optimus for a decade was "go stick your head in a pig."
You can't downgrade; it would have to be a fresh install. It doesn't sound like you've done that much that you'd need to redo, and the fresh install is, like, 15 minutes. You'll probably be fine on 21.04, though, since that's what you've got, but in general I'd recommend new users go for the LTS releases: the interim releases are for the testers and guinea pigs.
Having the proprietary driver installed is a necessary first step. You've not given any detail about how you were trying to do so. You shouldn't try to download anything from Nvidia's site, but you're coming from Arch rather than coming from Windows, so you'd probably have known not to do that anyway.
The repositories have the driver packaged up for Ubuntu, and there's a PPA that gets them before they go into the standard repository. There's an automatic tool that can pull from either of those, but I've never used it.
Update, It seems my option to start my session with Ubuntu on Xorg disappeared, I only have "Ubuntu" now which I'd guess to be Wayland, How can I fix this?
In Ubuntu 20.04, the session choices are:-
Ubuntu
Ubuntu on Wayland
Is it different for 21.04?
You can check which session you are using by opening a terminal and entering:-
Code:echo $XDG_SESSION_TYPE
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