NFP is simply a method of determining signs of fertility. Then it’s up to the couple to decide how they want to behave knowing what they know
Discernment is the element that is missing from this discussion. Op, it sounds like you have discerned that you have serious reasons to avoid pregnancy and welcoming another child. You and your spouse would then nNeed to commit yourselves to applying the most conservative rules
Couples that use NFP and embrace church teaching re: family life are allowing themselves to discern the question often. So to answer your question, I am 30 and am expecting my fourth child. In my marriage we have 1. Not ttHa or ttc; 2. successfully tta and 3. successfully ttc as our discernment guided us. serious reasons to avoid were no longer as serious with the passage of time (kids got older and more capable, meds managed medical conditions, savings accrued, etc)
Hope this perspective helps. (typing from an itouch, please pardon the typos and odd spell corrects)