Baby bjorn are the worst thing you can do, dangling a baby from their crotch is just not comfortable for them and is bad for their spine, hips and seat. I think they are pretty cruel, would you want to hang on a thin piece of fabric between your legs?
If you wanted a buckle carrier a manduca is a far better option and will keep your baby in the M, knee to knee position. They are suitable from birth if you use the newborn seat that comes with it, it is just an extra popper.
I'd use a stretchy wrap from the start though, a moby, boba etc., you can pick one up fairly cheaply. My baby is five months and 13 lbs, I am still carrying him in our stretchy though we now own more wraps than there are days in the week. I will be sorry when he is too wriggly for the stretchy.