Hi there,
I'm in the exact same situation. My daughter is 8 mo and 21 lb. I have several carriers and they all have pros and cons. So quick answer is try something other than what you already have, but it's not guaranteed that you will find THE perfect carrier. Keep trying, though...
Baby Bjorn synergy: We got it when baby was 5 mo when regular Baby Bjorn started to hurt my back. Lumbar support really makes a difference! Great for carrying baby facing out when there's things to see, i.e. aquarium. Compared to Ergo, shoulder straps are streamlined but just as strong. But since it only does front carry, it has a limit of 25 lbs. Baby probably will gain 1 lb per month, so if you get it now, you can use it for 5 months - it probably is not worth it, unless you can get a used one cheap.
Hotslings: I was really intrigued by the simplicity this promised. It is pretty good for hip carry and folds super small. But maybe it's me, or maybe it's my baby, but it's not hands free for us. I have to have my left hand on her to stabilize her. But it has its place. I mainly use around the house. Outside, I like to have both my hands free to carry a diaper bag, etc. I got mine used for $7! Definitely wort it. I don't think it's worth it to get it at full price ~$50. Seriously, you can make your own.
Ergo: I was debating getting Ergo and my DH bought it for me this week! Super excited about it. I was afraid baby won't like the split leg position but she actually likes it and was able to fall asleep in it a couple of times already. For me, if baby cannot fall asleep in a carrier, it is no good. Still need to practive hip/back carry, though. I think Ergo has the highest weight capacity I've seen (40 lbs). Have you tried different positions on Ergo?
I use maybe 45% BB, 45% Ergo and 10% Hotslings right now. All in all, I still feel the weight of the baby in each carrier. I'm beginning to think it doesn't matter which carrier you use, if your baby is getting heavier (of course they are!), you will feel it. You just have to keep trying different carriers/positions to minimize discomfort. Also, keep in mind once baby is mobile, they might not like being in a carrier for long periods anyway.