talk to your priest if you really are not sure, but generally speaking, most rational parents don’t have selfish reason for postponing for a while.
there are some people who say any sort of self-care is selfish, just ignore things like that. if you need a break, mentally, physically, if finances are tight, if you feel called by god to conrtribute your talent in some othe rarea of life, if your current children need additional care, all of those are just reasons.
I suffer from scruples, so I have often wondered such things myself, and honestly I don’t think I am called to marriage or parenting because just to figure out what is or isn’t sin just causes me too much anxiety but I would say try not to worry about it too much, just do the best you can, God knows your heart