Similar situation to OP's. Had 3rd at 40. Took longer to conceive, so the age gap between dc2 and dc3 was nearly 4y. Did not have PND after dc3 (but developed straightforward D a couple of years later...might have happened in any case). Dc3 took longer to conceive, and I had a couple of early mc along the way, which was why I had the bigger age gap.
My one and on ly regret about having dc3 is the bigger age gap. It has been very much like having two separate families. With a smaller age gap their needs are far more compatible. It's less rushed, too. It doesn't matter if you're late to nursery, but it does if you're late to school.
I have also found that I cannot 'wait for my depression to get better'. If I wait, if I put my life and my desires on hold, then it does not get better. Part of recovering from my depression is achieving things I want in life.
It's an up-and-down road. I wonder, does anyone ever truly recover from depression, or do we simply find coping strategies? I find it easier to accept, OK, I'm prone to depression. When things are good, I work on strategies to keep them good. When things are bad, I use those strategies and go to the GP for extra help. But I've got to keep my life moving forward, otherwise I give up on myself and things get really bad.