In fairness to the OP, it sounds like he is looking to take advantage of the several decades of development that have resulted in lighter, stiffer, but not bone jarring endurance bikes, and he is willing to spend some money to get there. Great offerings from any number of manufacturers fit that bill. My guess is any ride he takes on a Synapse, Roubaix, Domane, or the like will be an eye-popping thrill, and the only question will be picking the color and characteristics that best suit him.
If it is not that, then he can always resort to the less expensive option of fiddling with tires and tubes, but that won't make his lovely Mercx any lighter or the geometry more relaxed.
A category of bikes have been developed to answer the very questions being posed, and he should at least take a shot as seeing how well the developers have done.
FWIW: my Domane with 40mm gravel tires is more than competent on gravel, but significantly better than a dedicated gravel bike when fitted with 32s and chasing the youngsters up hills.