Yikes, I just started a long response and my computer ate it!
What is lacking in this picture is what strain of mycobacteria grew each time and how much there was (often scored 1-5 or as few, many...) If your lab doesn't do this (many don't have the capability, others only do it if ordered) you could ask for your specimens to be sent to the lab at NJH in the future. Many doctors/clinics do this to get a full picture.

As to how long to neb saline twice a day - obviously it is helping you - you are mostly asymptomatic and your lungs are not deteriorating- yay! Keep doing it. Here is Dr McShane has to say about airway clearance:

Should you treat? Here are the current guidelines from NJH:

Now the big question for you - to treat or not? This is a very hard one! Knowing what I do now about MAC and Bronchiectasis (I knew nothing in 2018) here is my PERSONAL OPINION. I would go with the "status quo" unless I became symptomatic with fever, cough, shortness of breath, all-day fatigue, frequent exacerbations (respiratory infections, increased colored sputum, etc) or worsening CT scans OR unless the doctor and an ID doc recommended starting treatment.

In my case, almost 7 years ago, that means I would have treated, even with more knowledge, and even if 7% saline had been widely used because I was severely symptomatic for many months, and my asthma was wildly out of control. Today I would be much more conservative and watch for three consecutive positive cultures that were not improving as well as other symptoms.

I hope this gives you some help in reaching a decision.