You said "Note that the US government has caught wind that NAC is beneficial, so they are beginning to regulate it" - but that is not exactly what happened. NAC is used (in high doses) in a hospital setting to protect the liver in cases of acetominophen overdose. The long-standing FDA policy is that if a drug is used by prescription, it cannot be marketed as a supplement, so they have challenged its availability over-the-counter several times, the latest in 2021. A few pharmaceutical companies chose to take their supplement off the market to avoid interacting with the FDA. Others have chosen to continue to supply it.

I have asthma and bronchiectasis (both predate Covid) and find NAC (N acetyl cysteine)in 500-600 mg capsules helpful in keeping the inflammation down in my lungs, which reduces exacerbations and makes breathing easier. There are a number of studies of people with bronchiectasis or cystic fibrosis showing it to help some people. It seems to be safe, so is probably worth a try if you have breathing issues. It is a low, safe dose, so results are not instantaneous - takes about a month, used twice a day, to determine whether it works.

Sue