First two hrs of the day and last two are the most productive. I’ve seen evenings as good or better than first light. Change speed and lures up to figure out what they want. Every day is different when dealing with staggers as they are picky!
Northern kings are a heavy spoon. A fish hawk is a game changer but without one I would rather be trolling too fast than too slow. I’d go 3mph speed over ground. Try and keep your lines pulling straight so adjust your direction to do so.