You could buy an onbuhimo aka onbu instead, they're soft structured carriers without waist belts, supposed to be pretty comfy. Can always resell via Facebook baby wearing groups if you decide you want a different carrier later.
I carried my 2.5 year old on my front in my toddler Connecta at about 6 months pregnant so you should be fine with a 6 month old! It's safe as long as it feels comfortable and in a few months your DD might be okay for back carries. I never really got the hang of them and my DD preferred being on my front although it does get quite heavy when they are older, but you should be fine until your bump gets too far in the way!
I carried my large and heavy toddler on my back daily until I was about 6 months pregnant, and occasionally after that. I found the connecta uncomfy on my bump though as the waist strap isn't padded, so I used my mei tai.
How about a ring sling instead? You can pick them up fairly cheaply (much cheaper than the structured carriers. Mine was £20), and could use a front carry tucking DD's head into the sling for more support whilst your bump is smaller. As you and DD grow, and her head control improves, you could shift her around to a hip or back carry. They are surprisingly comfortable, roll up really small when not in use, and the style means when used in a hip/back carry the fabric would run between your boobs and bump. In terms of how long you can carry her, it all depends on how your body is coping and if you're used to it. If you start now and do it regularly, that would cover the second point. When I was pregnant with my DD, I was carrying a backpack throughout the day for work which weighed around 15kg (it was 25kg at the start of pregnancy but it had to be reduced as I became less able), so it's do-able if your body plays ball