SmolAlli Is there any way for there to be a general app that would replace Google Wallet, that would allow for general purpose NFC to be utilised?
No.
The key issue is that Google Wallet is not a general-purpose NFC app. Google Wallet has many agreements (legal contracts) with specific payment processors -- and, based on this question, with some transport systems.
Any transport system that wishes to write an Android app that supports NFC payment can do so. It's expensive, though, and empirically a lot of transport systems figure it's easier to sign up with a small number of intermediaries (Google Wallet, Apple's tap-to-pay system, Samsung Pay) than to build and maintain their own app.
In theory anybody could write an NFC payment app that would interface with thousands of bus/train systems around the world, signing a contract with each one of them.
But there isn't a general way to import a physical payment card (bank payment or transport payment) into a general-purpose phone app without the cooperation of the service provider, because physical payment cards are deliberately designed to resist cloning.
This isn't a hardware problem, and it isn't a GrapheneOS problem. The problem is that companies (including government transport bodies) want to control how people can store value into their systems and extract value from them.