As strategic secession and capital movements are typically orchestrated to get troop recruitment closer to the military front lines, why not simply solve this issue by having recruitment centers in all of the duchies, with the only available troops in the recruitment center being those that were from each particular duchy?
I'm sure this has been suggested before (I mean, it's been 10 years or so), but it makes sense to me. I'm no history buff, but the time era, as I understand it, was really built around various powerful duchies operating together under a common banner due to a strong leader. Duchies would send their armies to fight for their King/Queen no? I assume those Dukes did not have to "recruit" troops out of a capital when the troops were trained within their own duchy...
So it would make sense to me if, and given the new tax code this might be a bit easier to do, Duchies worked much like mini-realms. Distance from the duchy-capital would certainly create more problems with control, which would be added to distance from the realm-capital as well. So that if Zonasa was fighting a war against Kindara and wanted to recruit troops from Azarons' cavalry center while stationed in Batesaor, it'd be really difficult to maintain or even do in the first place, because the Duchy would have to run through Hutael and Paplarmi, covering quite the distance and creating significant "distance from Duchy" control problems, on top of "distance from capital" control problems, which might make such strategies rather impractical.
Perhaps this has all been discussed, but it seems like it would alleviate the problems with moving a capital around for military reasons and strategic seceding.