With some luck (literally because of Idiot Savant) I've made it to Level 20 with no main quest and no visits to Diamond City. I've decided to tackle Nuka World next as that's basically one big side quest DLC.
I'm trying to do a Fallout 4 run while avoiding the main quest line as much as possible, which includes any of the faction quests and Diamond City is off limits. Honestly the hardest part is just finding side quests to do, besides a few name ones it's mostly settlement quests.