First: Less weight on the front wheel does not necesarilly reduce or increase wobble....it all depends. Wobbling and resonancies always depend on two different factors. Mass and stiffness. Mass decreases, the resonance frequenicy increases, stiffness goes down, resonance frequency goes down as well.
For the engineers: Omega(zero) = Squareroot (Stiffness/mass)
Also it is not one Frequency, but for every single degree of freedom one frequency and there are at least so many different degrees of freedom that are relevant as components involved....so at least Wheels, Tires, Fork, Frame and Rider. Since it may be lateral movement, vertical movement and even rotational movement, the number of relevant stiffness/mass ratios may even be much higher.
Then there are different factors that contribute to dampening. Tire, Frame, Wheel(s), Fork and last but not least, rider.
What human "resonancies" and "dampeners" can cause knows every bridge builder. One reason, why platoons typically don't "march" over bridges, but walk normally.
Most famous example of the later history was the Millenium Bridge in London, which had to be closed shortly after opening and additional dampers had to be installed, since the pedestrians would naturally and without awareness amplify the natural vibration of the bridge, since it's Eigenfrequency met pretty much the natural human walking cycle.
Anyway. For Fruitflys findings are many sound theories possible.
Too much weight on the rear wheel might infact lead to a negative outcome, since the front get's "too light"
What you could also try, would be to alter the tire preasure slightly, but on a road bike the effect is probably not as big as with an MTB.
You probably can get your Domane to the same behaviour....you just have to got faster
.It is important to understand, that it is not the "fault" of a single piece or component, but it's always the combination of all components that contribute to the vibration. An imbalance of the tire or wheel can go unnoticed in one bike, and lead to a "catastrophic" behaviour in another bike.
And it is not necerssarily better to have a stiffer Frame/Wheel/Fork... If they all have similar resonance frequencies, that's most likely bad. If some parts are tuned at higher, other at lower frequencies, this might actually be better. Also, the higher the resonance frequency, the higher the amplification and most likely the lower the dampening.
Most of you probably drive cars. Ever noticed how a worn out wheel bearing behaves? And what happens if you go faster?
So, it could be, that the Frame in general lacks stiffness, but it could also be, that the combination of all circumstances leads to the unwanted behaviour. So moving the saddle to the front, might actually be helpful, but most likely it is "only" going the shift the behaviour to slightly higher frequencies/speeds. Might be all you need and is definitely worth a try. You could also check what happens if you don't sit while descending. If it is related to your center of gravity, this should really make a difference
I'd also take different tires and pressures into consideration, as those can be swapped quite easily.
Which were the three different wheelsets that you already tried? Are these perfectly true?
With my old bike, I would get wobbling around 70 kPh, Ridley Helium with Lightweight wheels...not necessarily a fragile combination. Now with the BMC and other Lightweight Wheels I can coast one handed while eating/drinking easily at the very same speed at the same descend.
Really hope you find a working solution, because in my opinion the Escapee is a great bike.
