Vienna is a place with a great deal of alternate facilities
Yes, it is. But I cycled
to Vienna. Through places a whole lot smaller and with similar numbers of cyclists on tour.
you funnel riders say through an area where only a few accommodation options exist then you get locations where booking becomes necessary.
Well yes, and I don't think anyone is suggesting that. The third point on the OPs list of requirements is regular civilisation.
A long distance signed route is not going to pop up overnight. Lots of time to either reflect facilities in the route design or to encourage existing providers of a potential new market.
However, there is always the possibility that a steady stream of cyclists could actually be the catalyst for an expansion of services as mentioned upthread.
This is all pretty irrelevant to the OP, I think, but examples exist of just that. I'm thinking specifically of some of the Camino routes in Spain where the steady stream of pilgrims has kept many villages alive and given employment to locals who would otherwise have left. It's a virtuous circle as better services attract more people.
If riders are on a tour they’ve designed themselves and less wedded to “A particular route” then they can be flexible in their choices
Well, yes and that's just one of many reasons to create a personal tour as opposed to following one off the shelf.
It's why I'm curious why so many follow an "official route".
Bottlenecks for accommodation or transport isn’t just a UK thing.
In fairness I didn't see you address the transport issue and I can point to the relative ease of hop on and hop off trains in mainland Europe. My Vienna example was dismissed despite the fact that lots of bikes arrive and depart daily on the trains.
Yes, bottlenecks for services can exist anywhere. I'm struggling to see the relevance to a long distance route though. Generally, the services will grow to match the demand.
With the likes of AirBnB, just as one example, the market can adjust very quickly. A touring cyclist can have many options. Camping both official, unofficial and the "Camp in my garden" group, there's regular paid accommodation as well as the likes of Warmshowers and that's excluding any personal friends or family given that we seem to be discussing with the UK.
In short, I can see a lot of obstacles for the OP to overcome to achieve their goal but bottlenecks in services or accommodation are well down the list.
I suspect we'll see more "virtual" routes like the ones Cycling UK is developing, and like those in the fabulous Lost Lanes books - that is, unsignposted, but with wide circulation, and easy to follow if you have a phone or GPS.
I recently met a Dutch couple on tour following religiously a guide book by a Dutch author (name forgotten) who gave the most remarkable detail down to recommending individual sites in campgrounds or pointing out where exactly to stop to see something special. They were avid fans and had followed several of his routes (although it was far more than a route).
Interestingly, as well, there's a Dutch guy apparently driving around Europe in a van who collects bikes and gear and drives back to NL with them. They had arranged to leave their bikes and gear at a campground for him to collect in a couple of days while they took a train to Portugal. They have used him before.
Just one example of how demand can inspire a service.