If you want to spend as little time on your alt as possible - just do Legion Invasions. You can use sites like (https://wow.gameinfo.io/invasions) to track when the next invasion will be. Log in, fly to the closest world quest, and once there - right-click on the quest's name in your quest log and "Find Group". Sign up for some group, or if there's none - just start your own immediately, and people will come to you. It is 10 times faster to do invasion quests in a group. Once that particular quest is complete - leave the group, and fly to the next one, where you should find another group. Don't expect the group from one world quest to follow you to the next. Then you simply log out until next invasion. You can do 1-2 invasions a day, and each invasion will give you about a level or 1.5 of experience, if you have full heirloom gear. Each invasion will take about 20 minutes.
I'm very, very interested in a guide that takes into the following: 1) Dungeon finder; and 2) Heirloom items. I've never helped with a guide before, but would be open to help in collaborating on the project. With the leveling guides I've used (primarily this one) I, too, find that I level at a rate that throws the pace of the guide way off (not its fault at all, I can't say enough good stuff about the guide) to the point that I get frustrated and just queue up for dungeons.
Battle for Azeroth’s War Mode puts you into PvP mode and grants you 10 percent extra experience. But since the two factions will mostly level in separate zones entirely, you should almost definitely have this on when you’re leveling. That said, if you start getting killed by random PvP players consistently, stop using it. Although 10 percent is nice, it’s not worth dying every few minutes for.