First of all, only NFP may be used deliberately to make conception unlikely. Contraception or sterilization are not valid options. Just getting that out of the way up front.
The teaching of three Popes (Pius XI, Pius XII, and Paul VI) was that there has to be a “grave” or “serious” reason to use NFP. Some nowadays say that it only has to be a “just” reason, and there are even people who will tell you that any use of NFP is morally licit. Moreover, some will go so far as to say that, as long as immoral means are not used, a couple may avoid having children throughout the entirety of their marriage, regardless of reason, with the caveat that they will welcome a child if, despite their best efforts, NFP doesn’t work. (Of course they will! — what else would they do?) I imagine these same people would tell you that it is morally licit deliberately to have only one child — it makes little sense to say “no children is okay, but if you have one, you need to try to have two (or three, or whatever)”.
We have discussed all of this at great length on CAF. I have an inquiry out to the Congregation for the Doctrine of the Faith (formerly the Holy Office) regarding the moral liciety of using NFP deliberately to avoid having children for no other reason than “just not wanting them”, and whether NFP may be used for any reason or for no stated reason at all — not just “grave” or “serious” reasons. Given the CV crisis, I have not heard from the CDF, and I may never hear from them.
I have given this matter a lot of thought, and I am willing to concede that perhaps — and hang on to that word “perhaps” — “just not wanting children” or “just not wanting any more than we already have” could, indeed, be a grave, serious, or just reason for using NFP for the remainder of one’s marriage. Parenting is hard. Not everyone is cut out to be a parent. Not everyone is cut out to have a large family. Yet everyone has a natural-law right to marry, a right that predates Christianity itself — even people who are temperamentally unsuited to parenthood, or unsuited to bring forth a large family.
I don’t know those answers, and ultimately I submit joyfully to the magisterium. My advice to the OP would be to consult with their priest. As long as your reasons can be justified in some way, I can hardly imagine that a faithful, doctrinally orthodox priest in the year 2020 would give you a hard time over it.