Theory: King Il and Suwon are both doing everything in preparation for Yona to become Queen, albeit by different means.
I believe that King Il purposefully gave away land to make Kouka look weak until Yona was ready with the dragons to ascend the throne. Then like the King Hiryuu legend, she’ll go on conquests to get back that land and instill fear into neighbouring territories. A small period of King Il’s reign looking weak, will ultimately yield great benefit long term if the aforementioned plan goes through.
Suwon, knowing of King Il’s plan for Yona decides to do it for her for 2 reasons. 1) the irony that he is a descendant of King Hiryuu yet can not achieve the powers the dragons thus he is doing everything King Hiryuu did without the dragons. This lines up with the potential influence from his Lord Yuhon’s detest for worship of the dragons and King Hiryuu as well as to fight against the irony of his fate of succumbing to the Red Dragon Disease 2) because he cares for Yona and doesn’t wish to see her led wars where lives will be lost so he’s carrying out King Il’s plan and will just hand over Kouka at its peak glory over to Yona. This lines up with the fact Suwon doesn’t wish to be King for long, but just enough to achieve his goal of making Kouka great again. Also, he’s not overly ambitious, he only goes so far as to reclaim what was lost.