Hard to give advice when not in person. I have a Boba 3g and it is so comfortable to me.
I have to loosen the shoulder straps up before putting him in and then tighten till comfy, I don't mess with the side straps they are at their loosest.
Also you can try to wear him with his feet out and around your waist in the infant setting, like in this video. It helps them really snug down in.