I've run into my share of scammy taxi drivers.
Taxi across the town is £20, Uber usually 5-10. There are no other providers.
Taxi from my airport (some 15 miles away) is £60-80, Uber usually £30-ish. Public transportation (2 trains + 2 buses) over £50.
I wish I had an option.
Your aim is misplaced: ditch Uber, not GrapheneOS.
Every app on my phone has at least one other app, usually already installed, that can replace it. This wasn't intentional, it just happened naturally. Unless all two or three apps in a category get blocked for me at the same time, this already unlikely situation is barely an inconvenience.
If using GrapheneOS significantly increases the risk a person won't be able to use a service they rely on, that may be unacceptable.
I'm not doing this on purpose, I just now scrolled through my app list looking for one app that would actually fuck me up if I lost it in an instant. There are none. And I'm not currently even running graphene or anything else, just a stock Samsung.