Does Catharina find motherhood a totally fulfilling experience?
Catharina finds motherhood a fulfilling experience from several points of views, but not because she, herself, would be contented with just being a mother.
As a devout Catholic, she would have been fulfilled in using her body as a way to consolidate her family, and to increase it. As a wife, she is dully fulfilling her duties as the carrier of the heirs of the family. Also as a wife, she would prove with her many pregnancies that her husband had not shun her altogether, thus sending a subtle message to Griet that the husband (regardless of his attentions to Griet) still visits the marital bed.
Yet, Vermeer was not happy with his increasing household, had to convert to Catholicism for the sake of his wife, and needed the peace and focus that he could not possibly have had in that house so full of children, babies, and family members. Hence, it is unlikely that, aside from the points mentioned above, Catharina would be content with just being a mother for the sake of it.