Sounds like taxation to me...
Basically. Nevertheless the degree to which it is "imagined" or "real" is open to debate. In this particular setting we have had 400 years of Karagondization, so a great deal of Shayardene culture has been lost, and is now open to (re)imagination. That is something my MC intends to use to his full advantage, just as I expect the Laconnier's to do, regardless of their claims of somehow having secretly preserved the true Shayardene culture. I intend to borrow/steal that which is best from them, use the church and/or masses of helots as well as merchants and yeomen to check their ambitions where their interests diverge from what's best for the realm, and then if they become a threat as is likely, not only to steal the nationalist movement right out from under them, but turn it against their "self-serving lies".
I intend (hope) to reduce the number of generations (and bloodshed) required to accomplish this by co-opting the existing pillars of society and getting their buy-in, or at a minimum, their acquiescence as I remove the current crop of sell-outs to the Thaumatarch that run them, and place trusted lieutenants in charge who will perform a thorough house-cleaning. As for "bigoted and narrow-minded Shayardene nationalism", so far I've seen mostly scared Shayardenes and Wiends fearful that if they don't do what their oppressive masters from Karagond demand they will lose everything they have left. The truly bigoted will get sidelined to minimize the damage they are capable of doing.
Not a bad idea if you have a large pool of trusted and competent administrators who know those areas to rule them. As I mentioned previously, I'm already planning on partitioning Karagond myself. Given your plan to pretty much topple all of the pillars of the state, you may find it difficult to find sufficient administrators to carry out the partitioning you intend...
Without an outside threat, or a cultural glue like nationalism to bind them together, people will resist ceding local power to a distant, faceless bureaucrat. Just look at the conundrum that the European Union faces in maintaining its current power, let alone increasing it.
Whereas my helot has heard that there were no helots in old Shayard and the Xthonic scriptures that justify harrowing were added by Karagond's Thaumatarchy. So he places the blame for helot oppression squarely on Karagond. Oppressive Shayardene nobles and ecclesiasts are a secondary evil in comparison. The head must be cut off the serpent, and the head is Karagond's Thaumatarch along with his chief minions which includes but is not limited to his fake-Eclect theurges and the top levels of the church and nobility.
Given that Erezza is on the border with Halassur, and lies between Halassur and the rest of the Hegemony, any territory you surrendered to Halassur as part of a peace deal would be in Erezza, and giving up the easily defensible mountain passes sounds most unwise. Perhaps there is some land on the far side of the mountain passes that could be traded away without compromising Erezza's defense, but I'm reluctant to give something tangible (land) away for something ephemeral and easily abrogated (peace) by the other side, when there is no way to enforce the agreement. Every mile of Erezzan territory you give away brings the Halassurq Empire's troops one mile closer to your capital in Erezza, and allowing them onto our side of the mountains screams weakness and reeks of disaster. There's also the issue that ceding territory is politically very costly, and is likely to destabilize your grip on power.
"Minor border adjustments" are a possibility if they truly are minor and don't give away important defenses. Again, sizeable cessions of territory are likely to be politically costly for you, especially if you're putting your capital in the same province that's ceding territory, ie. Erezza.