Dear Editor,
I am 54 years old, 172 cm tall and weigh 75 kg.
I have a Pinarello Prince since 5 years, Chorus 11v, Fulcrum R0.

I ride about 5000 km a year with a lot of uphill.
Usually I go out once a week.
I'm not a fanatic of performance but a cycletourist pretty quiet, especially uphill.
I like to go downhill rather strong, especially on steep slopes and techniques with fast corners and hairpin bends (my record is 99 km/h).
Unfortunately, I often suffer from back pain after long rides (100 km), or a little more hard ride.
I'm thinking of switching to a frame for marathon, the most comfortable.
Certainly with Shimano electronic.
I am undecided for disc brakes.
The alternatives are:
Pinarello Dogma K - a bit too expensive either Dura Ace or Ultegra Di2
Pinarello Rokh - Ultegra Di2 - affordable price
Cannondale Synapse Carbon Disc - Ultegra Di2
Pinarello attracts me a lot and I am fond of the brand, but on the Rokh, on which the Dura Ace would be excessive, it would seem to go a bit back (compared to Prince).
Cannondale attracts me a lot for the disc and the price, although I am very puzzled about the quality of the supplied wheels (Mavic Aksium) that compared to my Fulcrum R0 seem much lower. Even the crank does not like me. Also I do not know if it would be worth go up in price is to choose the frame HiMod.
Last doubt about the groupset: it is well worth taking Dura Ace instead of Ultegra?
Thank you for your attention and I greet you cordially.

Signed letter

Well, the first thing to clarify in your case is that if from the change of bike you expect more performance, none of the solutions you have identified will enable it to achieve this goal and indeed, in many cases, is a real step backwards. Unfortunately, already possessing a great bike, improve it costs a lot. For back pain is to be seen whether this comes from the bike or from a not optimal seat position, and then to review, which I suggest you do before buying a new bike on which, perhaps, will experience the same problem.
Let's look at your bike: Pinarello, only DogmaK ensures greater comfort and performance, exclude the Rokh, a real step backwards. The Cannondale is even better than DogmaK, but in the Disc version has flaws that you yourself have noticed (wheels and crankset), although reliable and valid, but certain from a top range whell to an entry level ... is a big step backwards.
The Cannondale Hi Mod is lighter and if unites the Dura Ace groupset, the difference is even greater. Between Ultegra and Dura Ace only changes the weight, but of course in the end 200 grams less carry out their function.
So hard to give you advice; seen your problems of comfort and that you like downhill, buy the Cannondale Synapse Disc, conscious of its limits, even if you can always change the wheels...