Mazal tov on the birth of your baby!
It is permissible to push off mikveh night if you both agree. It is also permissible to immerse on time but delay relations, which allows you to stop keeping the harchakot and resume other forms of physical closeness. As we note in our article on mitzvat onah, it is generally preferable to delay relations rather than delaying mikveh. But this is a personal decision for each couple.
The Fertility Awareness Method of family planning (FAM) is most effective when practiced with the guidance of a qualified teacher, who can help you learn to track physical signs of ovulation. If you are breastfeeding, a teacher can guide you in a modified version of this method for breastfeeding women, which takes advantage of the the fact that exclusive breastfeeding tends to delay the resumption of ovulation postpartum.
For further discussion, please see our page on natural methods of family planning, which includes a section on halachic aspects of FAM. You can also learn more about FAM at this website, including this post on breastfeeding.
Please feel free to get back to us with any further questions.