We got a Littlelife XC (I think, old model) for our first (now 8) and are using it again for our second (1). It’s done service with the cousins in between and still like new. I can carry the little ones round for hours with no discomfort, well no more than lugging around a proper 30lb ruc sac.
We also have a Manduca, similar to the Gemini linked above. Find this is handy for front carry when younger and shorter trips, school run, etc. But is not as comfy/supportive as the structured Littlelife carrier for longer walks. Jnr also seems happier for longer in the Littlelife, I think he finds it less restrictive.