My 17 year old has severe long covid and fatigue is his most debilitating symptom but the mucus is right up there. It's so bad. He is now working with a functional med doctor and she had him do a GI Map stool test. Turns out he is positive for H. plyori as well as 3 other dysbiotic and overgrowth bacteria. And those 3 bacteria are also involved in mast cell activation!!! We go back in 3 days to talk about the results. I am hopefully that by treating his GI stuff that his mucus will improve. I think there is a huge relationship with the gut and overall health and would encourage anyone dealing with this to look for a functional doctor.
All tests done by traditional MDs (and we have been to LOTS) have been "normal" and there has been nothing normal for him in 2.5 years.