Your last paragraph highlights one of the affordability issues and whether you can afford more children. Or, to be more precise, the myth. You don't need to have a separate bedroom for every child. It would be nice to, but it isn't a requirement, especially when they are young. It actually creates more of a burden on the family, in that it is more to clean, more to furnish, more to pay for.
I didn't grow up with a backyard either. In the part of the world where I grew up, that wasn't a thing. But most families in Canada, consider an individual bedroom and usually some kind of lawn space a minimum requirement. You can argue that our culture is entirely wrong. Go for it. But you're not going to reverse the birth rate decline, if that's the plan.
