Going from 1 to 18-20 over a single campaign that will probably last 6-8 weeks Faerun time? Most adventurers don't reach level 15+ over an entire lifetime.
I think 1-11 is already stretching it. I would save 12-20 for a possible BG4.
I will always prefer solid logic in the game world over a player's need to get to max level and basically just get everything. High level characters need to remain rare and high level PCs also require very specific content. When they get powerful enough to threaten entire kingdoms and such. I don't ever want to see level 13 city guards or other nonsense like that.
