I swaddled Ds until he was 12 weeks as he would always wake himself by hitting himself in the face when not swaddled. For a few weeks I 'weaned him off swaddling' by just swaddling one arm (alternating for each sleep) until he was ready for a grow bag. Ds was an excellent sleeper and I defo think the swaddling helped.
I swaddled ds2 until he was 7 months! It was the only way he would sleep. I stopped when he had a heatwave and it was just too hot to swaddle him. That kind of broke the dependance on it. Swaddling makes them feel safe. they don't know that their arms and legs belong to them when they are very tiny and it freaks them out a bit i think. Swaddling makes them feel like they are back inside you. It will get better. Sleeping for 2.5 hrs at a time sounds within normal for 6 week old, well for mine anyway. It will get better.
We swaddled DS2 until around 10 weeks - by then he was managing to get out of his swaddle and was enjoying sucking on his hand. So one day we just did half a swaddle and the next we put him in a gro bag. It didn't have any effect on his sleep. Incidentally we tried a gro bag at around seven weeks and he would only sleep for 45 mins at a time so we went back to the swaddle for a few more weeks.
I was also worried about the developmental thing but I guess he let us know when he was ready to move on as I'm sure your DD will too.
I swaddled ds2 until he was 4 months old and only stopped then because we left the swaddle blanket at my mum's and a normal blanket didn't work. I tihnk you're meant to stop swaddling them when they can roll over themselves.