Hi, My name isXXXXX you for using Just Answer. I am happy to try and help you with your concern.
There is a great product on the market called ProFresh. It works by attacking bacteria that accumulates on your tongue.
You seem to be doing everything else right and since it is not related to sinus or what you eat, I think you should give this a try.
I myself have not personally used it, but I have a friend that did and loves the product.
Here is a link to their website where you can read about it and order it.
If you have any further questions, please let me know. I hope you have a good day.