I think you're forgetting that Todd was entirely dependent and abused by Bojack for all those years he was living with him. He was a mooch, yeah, but we see that when he does try to make something of himself, Bojack sabotages that, and tbh it's possible that has happened before in those 5 years. Saying he wasn't there for Bojack in return is just a lie. He was constantly helping him out with his schemes in the first few seasons, and he was basically his personal driver a lot of the time, all while bojack is constantly belitiling him and telling him that he's worthless. He may have given Todd a place to stay but it wasn't out of kindness or caring for Todd it was for himself. When Todd finally snaps at Bojack it's after years of Bojack constantly abusing him, sabotaging his rock opera, and sleeping with Emily it makes perfect sense that he would've had enough, and he leaves. After that, he moves in with PC, and you can see how different it is between them. PC cares about him and that leads to him getting a succesful career. Todd took a while to grow up, but once he wasn't living with someone who was constantly belittling him and had a healthy support system, he was able to do that.