If you mean those ro.product.*, ro.system.*, ro.vendor.*, etc. and some other device identifiers, I think you can adjust many of them when compiling GrapheneOS yourself, which, however, requires a lot of storage and computational resources. If you mean the user agent HTTP header (and navigator object), there's no such thing as a system wise UA since UA is to recognize the OS and browser combination, and the UA header can be spoofed with those user agent switcher extensions or, if you are advanced user and want fine control, Header Editor, which can be added to Android Firefox by visiting it with desktop enabled.