@equanimous RTs seem in short supply, like so many other positions today. I could be wrong but I thought there was a highly regarded pulmonary practice at one of the big medical centers in LA. RTs not only treat lung but also heart patients and others, so I'm surprised you can't find one. My experience is that they are difficult to get an appointment with unless you need pulmonary rehab. etc.

In the meantime could you check out Buteyko breathing, used for all sorts of things. There's a best selling book called Breathe: the New Science of a Lost Art and others, and various websites. Very interesting and possibly helpful. There are Buteyko teachers scattered about but not covered by insurance.
https://www.buteykobreathing.org
https://www.verywellhealth.com/how-do-i-perform-buteyko-breathing-200874

You might ask to talk to a RT with Hillrom. I have a Smartvest but not using it much because of my history with spinal leaks. I'm deciding whether to send it back. I'm working with the RT with Smartvest and she is very good. She isn't just telling me to keep using it but helping me to work through the decision considering my history.