Every situation is different, but I will tell you my experience if it is helpful in any way.
I waited after my child was diagnosed. I felt exhausted and harried, and genuinely felt I was doing the right think for my special child. But in the end my next child is 5 years younger than her, and I do regret waiting. Of course I will never really know what would have been, but I see how having a younger sibling sooner could have been an enhancement for my sn child rather than a detractor.
I regret not getting more support for myself at the time, which could have helped me to feel ready sooner.
I also have the fear of another one.
My challenging child takes up so much of my time and energy its hard to squeeze out time for my other children, and make them feel equal.
My challenging child is getting easier as they get more mature, waiting a little longer until I add a demanding newborn to the mix
My oldest is severely autistic, he's 13 and still in diapers, he has minimal language, he became very aggressive and at age 9 and a half I had to look for a family for him...
My other 4 kids are doing great no issues whatsoever, the top of their class socially and academically! I'm very happy I didn't stop or take a break as we now have B"H a very happy and healthy family!