Welcome back. I found that leveling in retail in dungeons is a pretty poor way to level. Questing is almost always more efficient. When I came back I started boxing classic, got to mid 20s, got burned out, starting boxing retail. Had an absolute blast, but then Shadowlands was announced. As a result, because I believe going from 1-50 in Shadowlands is going to be ridiculously quicker than 1-120 is today, I've switched back to Classic and honestly I've been having a blast since. I'm sure in the 40s and 50s I'll get burned out and I'll begin working on my retail team again. I think you could do multiple classic teams to break up the monotony, or switch between retail and classic.

It really depends upon your goals in both classic and retail. The goal for me is to deck a team out in pre-raid BiS in 5 mans in Classic, and probably never play again. In retail, I'd like to level a few teams up and get ready for Shadowlands, knowing the meta could be completely different than it is today in M+ and I really don't have the time to devote to the game that I would need in order to level a bunch of teams to 120 to accommodate that. It would be nice to get to a point where I can self fund my subs through WoW tokens in retail, but I don't know that I have the time to accomplish that AND complete my classic goals before Shadowlands.