Following this line of reasoning - a person being to uncomfortable to enter a leveling rouletter and doesnt has friends to do it with them - can you explain why that person would want to level a second (or third etc.) class at all?
Leveling the first class happens by just following the Mainquest, doing sidequests and doing the dungeons required for the story. There is little to no extra leveling involved - so someone who doesnt like running dungeons and only wants to see the story can already do that by just following the story.
Why would you need additional classes at max level if you dont even feel comfortable running dungeons? (And why are you playing an MMO if you havent got used to running dungeons after well over 200 hours of playtime and at the very, very least 19 dungeons - because thats how much it should take you to get to SB?)
I see no real need to alter the exp-output. There are many, many ways to get a decent chunck of exp for your battle classes - if you dismiss some of those, thats on you. And if you dismiss the ones that actually prepare you the most for using your class in level 70 content, thats even worse. Remember all those tanks that leveled by doing fates (and couldnt tank a dungeon in the end)? Or all those people who get easy exp in PotD, but no proper experience on their class? Leveling outside of dungeons doesnt need to be made easier when most of the actual battle-class endgame is happening inside dungeons - the leveling experience should prepare your for that - I'm exaggerating a bit now, but I dont want to enter a level 70 dungeon one day to have the tank say "First time tanking a dungeon!"
The game will let you avoid dungeons for the most part for leveling one class - that will allow you (theoretical) access to the whole game-content. If you want to have another class at 60, you gotta put the effort in. Exp are cheap enough today. If you dismiss some of the best options - thats on you.