There's not much patchworking/assembling once you bookmark your preferred locations, research what times they open/close, select favorite transit modes, bookmark specific lines, research which exact stops. Don't mentally lock into Google Maps as your primary thing.
I said the c) part is already done by Transit App and generally better than Google Maps (shows rideshare time-and-cost estimates, bikeshare/scooter pickup locations etc.) Where 'taxi' has generally been replaced by rideshare and by 'private bike networks' you mean bikeshare. Not sure if by 'private bus network' you mean coaches, or employee shuttles, but those are sometimes not open to the public.
As mentioned, the amount of local detail depends on how much your municipality participates in open-date initatives; if it doesn't, ask them to. Transit App is adding new regions constantly.
You're still left patchworking all these apps, switching from one another at every point and assembling everything in notes by yourself. It's super nice it can be done at all, but I'd also pay for an app that has it all in one place if it wasn't free.
To note, the c) part is not just public transport but anything from private bus network/taxi/private bike networks/just walk. While Maps deals with that decently, I'd agree it would be way too much too ask from a free transit app.