Why your seemingly destined Enemy would need the Tribes of the North to be alive matters little you suppose, and you could always just kill them and be done with it should they turn out to be more important. So at least for now you decide to focus on the Ironborn, seeing as they make for a much simpler and overall more entertaining Topic to think about, especially now with how so much of the Ironborn's Steel is in the Hands of Wildling Tribes, yes, you're quite sure you can finally have some fun again.
Going by the Reports on your Father's Desk, the Fleets of the Ironborn were once more sailing normally, with the Exception that a number of them were seen sailing north, most likely trying to rob the West Coast of every Tree they can get their Hands on. It seems like looking at the odd Kraken appearing before the Westerlands Coast combined with the marshalled Strength of House Lannister kept them at Bay like you had originally planned. And as boring as these Weeks of Peace were, you had to admit that they also granted you a better Understanding of the Logistics surrounding a Nation dependent on the Sea.
For the Ironborn simply don't have the Soil to feed their Population, meaning each Ship they risk in the plundering of the Forests owned by the Starks is a Ship that can't supply the Population with Sustenance for Weeks at a time. A balancing Act that as far as you know requires Knowledge of the Sea unrivaled on the Continent, which you can more than respect considering their Culture of Arrogance. Their Lord just like your Father seems to be a Man you can happily call a proper Human without having to adjust to their way of seeing the World, perhaps you could convince your Father to help out the good Lord Quellon, one can always respect Competence after all.
You guess you won't steal their Iron the first Chance you get after all, at least if things keep proceeding as they were they could turn out to be truly useful in the Game of Thrones, as some of the Madman in the Capital called the struggle for the Crown. And hey, your Family needed a bigger Fleet for a potential Campaign anyway, so why not make some Friends that could help your House transport the Supplies the Armies would need, once they were created?
An open Alliance at this time would of course be out of the Question, and you're still quite displeased with their God, which you're sure your Father hasn't forgotten either. For now the Ironborn simply had to suffer for a bit, so that the Westerlands could continue to mobilize and expand its Armies in the Rock, away from the prying Eyes of the Crown, who now had the pleasure of finding the Fleets of the Ironborn that moved to raid the North, which isn't half as easy as it sounds.
For Ships are simply faster than Horses, and with House Stark having no Fleets at the ready, and the Royal Fleet being on the other side of the Continent, it would be neigh impossible to keep track of their Ships, especially seeing as the Crown wouldn't know or believe that a great number of their Ships were destroyed by a Kraken. No, not with the current King, for he wouldn't believe a single Word of it no matter what Source might deliver the Reports, such are the daily problems of Men who serve under a Lunatic after all.
And with the Situation worsening by the Day, you're confident that even the Starks will begin to have their Thoughts on the matter, not that you expect them to do much against the Crown with the North being invaded from the north and the west by Warbands. No, you'd prefer if you didn't hear of them again in the Decade, for the Stone from the Wildlings was normal, meaning throwing whatever is beyond the Wall back should be possible even without the help of the great Houses, surely at a steep cost with how long the Winter lasted, but possible nevertheless.
Thinking for a moment on how to proceed while the Realm deals with the Ironborn, you would say that the Reach could make for a good and juicy Target if its Lord takes after the King in matters of raw Ability. For just like the Iron Islands the Lord can make all the difference in the Reach, and you seem to be in Luck going by the lacking Defences despite quite the number of Ironborn Fleets being unaccounted for.
Now the Question became when to strike, namely before or after the Iron Islands were inevitably burned down. There would be the Option of sinking the Royal Fleet after they finished their Invasion, or if an early Alliance between the Rock and Pyke could be put together you could simply stop the Tyrells from ever amassing their admittedly massive Armies, picking them off a few thousand at a time. But sadly it all depends on the Ironborn enduring a long and drawn out Siege of their Islands, and while it's not impossible, you honestly can't see your Father agreeing to a Plan based around the Competence of the Ironborn.
The Realm while sadly still largely at Peace was at the end of the Day satisfyingly split, just like you wanted, House Lannister against House Targaryen, and House Greyjoy against the Crown and House Stark, if the Troops were ready or another great House would take up Arms against the Crown you could finally have your open War. So very little is still lacking to push your Father over the Edge, and you know like anyone else that the King knows it too, hopefully you will learn someday what the King actually did to earn the Ire of your Father.
It would be rather interesting to learn what made the Lord of the Rock so let's call it enthusiastic when it concerned potential suffering for the current King, but no one in the Rock seems to have a Death Wish, with even your Mother being awfully aggressive about it. Mayhaps you will one Day get the Chance to put the Man in Question into one of the many Cells within your Home to ask him in Person, for if the great Aerys Targaryen managed to actually infuriate your Mother, you don't think you will like his Words any better, giving a great Excuse to have some fun with him.
You can find this Fanfiction on YouTube, if you're interested