Indeed, by the time you have 3 fortress with prison the game is already won, even in expert/ridiculous/insane (especially on insane).
A trick that i'm willing to share regarding gildar spending is to rush creature dwelling without killing the guard, work best for buying dragons without killing the dragon guarding it, you can kill it later with your own dragon after you do the trick.
How to do it? Click the dragon guarding it, then about 1 second later click it again, if you do it too fast, you'll double click the dragon and bring up the statistic window of that dragon, if you do it too slow, nothing will happen or it's just like clicking the dragon once, but if you do it right, you'll click the dragon lair, and the option to build or raze it will be present as well, then you can build it if you have "dance with dragon" tech, and then on the city that build it, you can rush it, then you'll receive your dragon immediately. Though the dragon guarding the lair (which already become a dragon camp now) is still present and will sometime try to raze the camp, but it doesn't matter, because if the camp is razed your dragon won't disappear.
I don't know if that is a bug or a game flaw, but it already present since FE v1.00.