I understand what feeling stretched feels like. My first born was diagnosed with cerebral palsy just prior to the birth of my second child. We were attending physio & hydrotherapy classes every fortnight whilst also attending a special school three times a week. I was lucky to have great friends and family that helped. Had it not been for financial reasons we probably would have had a third child, but only you can make that choice.

My eldest was a bit of a nightmare baby. Obviously nothing like what you have had to deal with regarding an on going health issue. I had bub number two 3 years later and she was blissful. Bub number 3 was another 5 years after number two. There are no set rules. If you are not ready then wait.

It is hard to advice you in this as this is an decision you and your partner really have to make yourself. It all depends in finding a balance , whether you have a hands on partner, whether you have family nearby to lean on for support etc. I understand you feel stretched between juggling everything in regards to the health of your eldest.
I remember the time my 2 youngest kids had weekly specialist appointments and therapies and without family around for support we often also had extra foster kids in emergency or short term care. Yes it was busy but by going with the flow we managed well and my kids loved their siblings and extra foster kids in the house.