Did the ENT discuss with you the possible causes of your sinus/throat problems?
When I hear of problems like yours, I think of allergies, infection or a cold. If the ENT say no signs of infection and if you don't have symptoms of a cold, then you might have to consider allergies.
If allergies, then you might want to try and lessen your allergies through allergy tests and then allergy shots, aka: SCIT (Subcutaneous Immunotherapy); or allergy drops you can do at home, aka: SLIT ( Sublingual Immunotherapy).
Also, you might consider finding if certain foods cause your sinus misery and avoid those foods.
I found out that I was allergic to milk products. When I stopped consuming milk products my allergies and my health improved quite a bit. I also had to avoid many other food items, but that is another story.
I don't know much about your Sinus Rinse, except to caution you to avoid getting water trapped in your Eustachian Tubes located near the back of your throat. If you get water in your Eustachian Tubes during the Sinus Rinse, it could give you a world of hurt!
Best of luck getting your sinus and throat problems under control; and your singing career back into high gear!