For those not familiar with the term: You position yourself with your head a foot lower than your hips (and your back straight). You hold that for 15 minutes or so. You let gravity pull the junk out of your lungs down to where you can cough it out. Warning: It's not fun. First, your head feels like it's going to explode from the lung pressure. Second, when a chunk comes out, oxygen starts hitting an area of your lungs for the first time in maybe months. That hurts. (Or so my daughter says. She had to do this.)
"Postural drainage is considered safe and effective, but may cause some side effects. The procedure is discontinued if the patient complains of headache, discomfort, dizziness, palpitations,[3] fatigue, or dyspnea. Patients may be dyspnic after the various manuovers, since the head-down position increases the work of breathing, reduces tidal volume, and decreases functional residual capacity (FRC).[citation needed]"
The particular FB group I'm in is called (creatively enough) "Long Covid Support Group", but a search there reveals there are many more.
My email is in my profile if you want to connect further over this. I consider myself mostly healed by this point (infected March 2020) and can share my vitamin regime (its efficacy is anecdotal, my sample size is 2) but I still have lingering health issues over a year later.
You may want to look into the Airofit Pro: https://www.airofit.com/
Funnily enough, I used to play the flute and sing before contracting COVID. With my lung capacity halved, my ability is nowhere near what it used to be. Long COVID has taken all my hobbies away from me: running, singing, hip-hop dancing. If anyone knows of any support groups relating to Long COVID, please let me know.