just a thought - I use my hugabub for hip carry. my girls are pretty small for their age but they are 9kg so sounds like they are bigger than your DS at the moment.
I know you are only supposed to front carry with it but I have had a play around with mine and have found a really comfy hip carry. Worth a shot. Have a look at baby k'tan website and look at the video for hip carry - that is what inspired me to use my hugabub like that.
Have also heard good things about ergo. Was going out to get that when I bought my hugabub. the sales lady talked me out of the ergo. kind of wish I had got it now but never mind...