Which is based on Android, a Google product. What if I don't want to run Android or iOS? What if I want to run something that isn't Linux based?

I realize this is true. This is more of a personal choice since apps like Venmo can still track you on Graphene:

GrapheneOS does not strip proprietary tracking code embedded directly inside third-party apps like Venmo. If you log into Venmo, the app collects your financial activity, device analytics, and interactions

Venmo requires a verified account tied to your real identity, phone number, and bank account, meaning the service already knows who you are regardless of your operating system.

Venmo can see your IP address and any location data you permit the app to access, or data derived from your connection