Failed to explain why you’d use the LFD when you stated it, also lvling in retail is also a joke what with the 700%+ exp bonus you’ll go through most if not all content pre BFA like it’s nothing, then say in island expedition from 110-120 and knock those lvling out in a day or two. That’s only around 1-2 weeks of grinding.
Boosting you can only “live in dungeons” from SM-ZG assuming SM carry groups purposely go slow (because gear speeds the kills up).
Also you can always get a lvl 120 to run a lower lvl char through a lower lvl dungeon as they can one shot all the mobs in there anyways, the 120, and you have to run to the physical dungeon (hard I know what with flying and 2 seater mounts and all).