I learned NFP while I was pregnant with my third. I felt comfortable relying on LAM while breastfeeding as my only “method” (if you can call doing nothing at all a method), only because I had a history of not getting my fertility back until the infant was 1 year old. That is all I have ever used in the first year after a baby’s birth. I have used symptothermal NFP once my fertility signs show up again, first the mucus patch rule and later cross-checking with basal temps.
It worked very easily for me when I needed it because I had very clear signs and reliable temp readings.
I bought the monitor for Marquette and never used it because STM worked so well and it was free.
I am sure that some people here at CAF will give their experience which is likely to be very different from my own, but everybody is different. I have friends who used the old-fashioned calendar method and it has always worked for them. Go figure.

Good luck with finding what works for you! And God bless you and your family. Congratulations on your new baby!