You want to be a crafter?
You will level slower.
Accept it.
Even with higher xp for crafting, you will level slower.
You will run into both gathering and crafting cooldowns.
XP for crafting is fine.
One of my alts reached level 6 this weekend with questing and crafting only.
BBs are nice for crafting, 24 junk in 1 run makes 6*40 xp and 1200 caps worth in BBs.
Which I actually sold for caps in NCR, the Hub, Junktown and the Boneyard.
Pure crafter builds are supportive builds.
They don't need to be level 21.
Crafter/fighters are just that, crafter/fighters.
They will have a harder time fighting than the pure fighter build, but still they should be fine.