I just put the villagers who are almost at a new level in my cabin, always build amenities I need (that I have resources for for all levels) and just use everything I got - using every treat to level a villager up (i.e. never waste a treat on someone who won't level up from it unless I use like 6 gold treats on a new villager) and use every calling card and request card to get the other villagers closer to leveling up. I think most importantly I always used leaf tickets to complete villager treasure maps so that I get all of the treats. The maps give out a lot of treats through that and now that the game buffed gold treats you'll get a lot of levels out of doing that. Just make sure you don't waste treats on a villager who is locked in at level 15.