I’ve always thought Felipe and Letizia would have had at least three children had they not had unique pressures at the time of Leonor and especially Sofia’s birth. I think it might be an easier family dynamic when there’s more than one ‘spare’, as opposed to three members of the family who have their lives mapped out and one who needs to know from the start her life will be different. I also think, as much as he clearly loves his girls, that Felipe would have enjoyed having a son.
I don’t think any future Spanish government would object to limiting the working members of the royal family, so to me it would be best if Sofia was prepared for a fairly normal, albeit very privileged life, right from the start. Similar to what her aunts and cousins have now. That way she can fully devote herself to making the life that she wants in terms of studies, future career, where she wants to live, etc. And, at the risk of sounding like my grandmother, it would probably make dating and finding a good man to settle down with easier, too.