Don’t overthink it
I think this is good advice for most situations in life, but not when it comes to having children. I think, if anything, more people should think long and hard about the possible outcomes, good and not so good, when deciding whether or not to add to their family. There are few decisions in life that can't be taken back or changed in some way, but this is one of them. I work with a lot of families who seem to take the "don't overthink it" approach to having more children, and end up struggling financially, emotionally, with balancing work and childcare or to give their older children the time and support they need.
OP, you mentioned additional needs. We always thought we'd have 3 children. But then we had 2 with SEND. They are wonderful and I wouldn't change them for anything, but life is not always easy, there is a surprising amount of admin involved and we have to parent very differently to our friends with NT children. Our eldest wasn't diagnosed until 7 and our youngest at 4. I am forever grateful that we didn't make the decision to have a third child before we really knew what we were dealing with in terms of their needs, which have become more apparent as they've gotten older and the gap between them and their peers has become more obvious. Financially, it would have been doable but we may not have had the options for private therapies, assessments etc that we have sourced for our DC without having to sacrifice things like holidays, days out and other fun stuff. In terms of our relationship and mental health, I think a 3rd would be a disaster.
Even if you have 3 easy children with no additional needs, adding more children is inevitably going to mean less time, less money and less energy to devote to the children you have now so I think you're right to be giving it serious consideration. I would ask yourself, what are you going to get from a 3rd child that the 2 you have now don't give you?