We have our 3 month old in the buggy seat reclined flat :)
We've moved ours to the pram seat for the same reason - he's the size of an average 6 month old so not really sure what else to do. I keep it reclined and try not to keep him in it for too long without short breaks.
I’ve got the same pram and I’ve just moved him into the seat rather than bassinet. We use it reclined and I also have a sheepskin insert which I brought separately which means he’s very snug and supported in it.
I’ve moved my 3 month old into the stroller today too she’s too big for the carry cot! It reclines flat but her head control is quite good so I had her up a bit so she could look out and she was supported by the shoulder pads on the straps xxx
I have an older ickle bubba and the seat reclines flat so we will use that. I got a silver cross wave but the seat is way too straight up