@kahkej Welcome to Mayo Connect, I see you have been "hanging around" here for a while but this is your first post, and it is a really good one.

Lets' start with your last question first "...I’m confused about why I am susceptible to NTM and wonder if it’s always in my lungs waiting to cause havoc..."
Your lungs have a condition called Bronchiectasis - a disease that disrupts their function and leaves you susceptible to infection. Not only NTM (also called MAC) but Pseudomonas and other bacteria and viruses too.
Here is a good explanation of bronchiectasis from a respected source:
https://www.bronchiectasisandntminitiative.org/Learn-More/I-am-a-Patient-or-Caregiver/Bronchiectasis-FAQs
If you have not seen this excellent explanation of NTM, take a look at it:
https://www.ntminfo.org/wp-content/uploads/2019/06/NTMSupplementalGuide.pdf
It sounds like you are on the right path doing airway clearance, and the saline has been shown to help "tamp down" if not eliminate NTM over time for some people.

The only concern I would have is the short period of time you took antibiotics - this does not follow the usual treatment protocol, which is to treat with 3, and sometimes 4 antibiotics until 2 negative sputum cultures, and then for an additional 6 or 12 months.

Do you know how familiar your pulmonologist is with treating Bronchiectasis and NTM?