We have a Tula which will last through into Toddler days and I find it really comfortable, as does my husband. Check a local sling library as they will have some different brands you can see and try before investing though - it really helped us.
I have the ergo baby omni 360 which is very comfortable
I thought the baby Bjorn could take 15kgs? If it's the same one as mine then it can also be used as a backpack carrier which will help your back!