So, back story is important right? It fleshes out the characters and we get to know them better. I guess back story doesn’t happen on the first draft, but on the second, when you try to make the story more coherent. I have a bad habit of making stories out of back stories, meaning they can be read as a whole different story, and is sometimes, unrelated to the first one. I mean, that’s how you discover the back story right? The same way you write your first draft? How else would you do it… I guess the problem is that the back story sometimes is more, or equally, as interesting as the story itself. Does that mean the first story is flawed in some way? Because, back stories aren’t supposed to be that interesting… A back story should be sprinkled out and shown throughout the story and perhaps my problem is that I’m over developing it. It’s fine if the only thing I know about the main character is that he ran away from home because he came from a dysfunctional family and that his father didn’t want children to begin with. I don’t need to go deeper than that, do I? I guess I get easily carried away ones I begin.
It's better to write interesting passages than boring ones. The reader won't mind reading anything if it's interesting. I'd try to minimize mundane details. There's no need to include a life-history or a CV. They're not applying for a job. If a character speaks or behaves in a way which would seem strange or irrational to the reader, some backstory would be helpful to explain why they have that characteristic otherwise, the reader may think you've just written the character badly.
If your back stories are as interesting as the story itself, that is really good! I think you should combine them and, like you said, sprinkle and show them throughout the story. From what I've learnt, back story should be kept to a need to know basis. If we are told things too early, maybe we will forget it when it becomes important or relevant. And if possible told indirectly, through environmental storytelling. I guess your choice of viewpoint also affects how much and how fast back story is best revealed.
It depends on your process. I workout the backstories for my important characters before starting my first draft. Same for my worldbuilding. It needn't mean your original story is flawed or unworthy of writing, but it could mean that. There are many factors that you might weigh to determine which story you should tell, not least of which are your own preferences, enthusiasms, and intuitions.
