The companions have backstories, but I wouldn't call them "epic adventures." It's more like they screwed up their lives in various ways and got some powers on the side, but haven't done anything with it to make a mark in the world.
This is their first real adventure in D&D terms. The adventuring is what drives the leveling. I don't have a problem with it personally, but then I've played so many games like this that I don't mind hand-waving some of the setups. I'd rather be adventuring with this group than with a farm boy.
Well, Gale was such a prodigy he had sex with Mystra, the goddess of magic and after she ignored him he found a Netherese artifact left over from Karsus' Folly, the only 12th level spell ever cast where Karsus took over her body, the weave undid itself, she committed suicide, and a new goddess was born to take over and repair it. Gale then had this shadow weave chaotic magic from the time of days of the most powerful spells cast and items created and it somehow consumed him when he opened it and now has to feed it powerful artifacts or he thinks that his explosion would wipe out the largest city on the Sword Cost, Waterdeep.
Gale kept saying that when he lost the favour of Mystra his magic suffered, then he went out and just bought that book with it's bubble dimension before having the magic claw his way into him. I think we're supposed to believe that losing the (raunchy) favour of Mystra AND having a weave eating magical parasite in him made him weak as a newborn kitten. So I will raise my eyebrow and play along with that.
Otherwise level 1 seems fairly appropriate for the rest:
We have a somewhat lazy vampire who sounds like he did housework (or also sex work, who knows).
A folk hero that did some stuff we've apparently heard of but aren't super impressed by. Also he has a demon patron that could've given him an extra little boost to put him in the right circles if it was something impressive.
A Shar cleric that's core competency is being surly and acting like we should be surprised she worships Shar.
A Githyanki that spends all her time talking about how she's going to prove herself and how she followed protocol like it's her first day on the job.
