How much do I gain from switching to it instead of say, remaining on the Stock Android?
Edit: This looks comprehensive — https://staging.grapheneos.org/features
Beyond that, most of the other advantages will be less visible. They have hardened memory allocators that make various classes of security beaches significantly more difficult. There's a lot less superfluous background services eating resources. All that and more are listed on their website. It's well worth a read.
Remarkably, Nationwide (UK) runs perfectly even without Google services. (Except it doesn't poll for payment confirmations but that's understandable. They're always fetched when you open the app though. Or it could be my setup). It's actually quite shocking and it speaks to Nationwide as being a decent organisation.
It's also not necessarily a downside. Having your banking app on your phone can be a risk of you often take your phone out in public
The absolute blocker is Google Pay. That isn't supported