I think a less ambitious strategy would definitely increase the odds of success – it prevents over-extension, allows you to concentrate your forces in a more defensible location and keeps your supply lines much shorter and less vulnerable to attack, and means you can avoid the hazards of the open deserts.
I do think it has been tried in the past, but the reason why it hasn’t worked very well is that they weren’t able to divide the enemy against itself. The Yronwoods aren’t about to ally with the Dondarrions, because after thousands of years of war between the two Houses, they’re not going to trust one another. Likewise, I would imagine there’s a lot of bad blood between the Gardeners and the Daynes and Fowlers.