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There is orbit360 in Germany, basically what you describe. Seems like it's 29 routes now, supplied by different people and you ride them in a certain time frame, record, and someone is fastest. Started as a ride during Corona thing.
If I remember correctly all the start points are close to public transport.
And it's also definitely inclusive as the routes are available and then you just go ride.And there's gravel brevets around Berlin now, will probably ride some next year.
I don't think it's lack of imagination, it's just the ultra omg so hard thing ultimate whatever that makes people think it has to be in a far away place, high mountains, the north cape.
Succinct enough I guess.