Some tips regarding my experience with this campaign.
I suggest trying this on the Convergence Kingdom map, because the map is small with many nodes that connect to four locations. This means you can reliably complete the final quest step with plenty of time to spare before the 45 day limit expires.
*** Spoiler - click to reveal ***The final step involves clearing the Curse off all regions in the map. This is done by arriving at a location with the purifier stagecoach item equipped. Once you arrive at a camp or inn, all adjacent regions will be cleared of the Curse. On the Convergence map, this can be done easily in 4-5 days, whereas the bigger maps are going to make you travel far to clear out every little corner.
Additionally, with the small map, there are fewer inns, making it easier to get tier III militia in all of them pretty early on, with plenty of chances to arrive at capstones to help secure the late game.
The one downside of this strategy is that this map has no two inns right next to each other, so you always have to go through a camp (besides underground travel). This makes the early game a bit tougher. I restarted a few times after losing heroes too early on but eventually, once I secured the early game and had a reliable team, it was easy sailing through the later parts.