The prohibition has never been on moving the capital for strategic benefit—as you say, nearly every capital move has some strategic benefit.
It is specifically on moving the capital for the strategic benefit of recruitment closer to the front.
And, as was complained about a certain amount recently, it is at least partly a crime of intent. The same action can be considered either definitely fine or definitely illegal, depending on what people have discussed and stated as its reasons. (Though most of the time in cases like this, it's somewhere in the middle...)
Given the situation you have described for Highmarch, I would be highly skeptical of any capital move that happened in the near future. Frankly, I'd be highly suspicious of any capital move that happened less than a year after the creation of the realm unless it was to move the capital away from a front that had suddenly come to it (due to regions being taken by enemies).
Consolidation of military gains is something that most generally happens during peacetime. There is an important line to be drawn here between this sort of thing and the secession that led to Highmarch's creation in the first place, as a) secession inherently creates more opportunity, and b) secession removes regions from a realm, which can quickly alleviate the serious pressure that being too large can place on a realm's regions. The pressures of being far from the capital tend to build up more slowly, and are by their nature not shared equally across the realm.
...I hope that sounds more coherent to you than it did to me... 
I learned that Akesh Temple was meant to be the initial capital of the realm, given it is more central, but that when seceding the game forced the selection of a city (why?)...? Would this factor in a decision to move it to Akesh?
To be clear, I haven't heard of the ruler talk of it yet, other than maybe mention he'd fear it against the rules. But as the new lord of Akesh Temple, I'd sure love the capital to be there, sure would make my life easier.
