Can oral thrush spread to the sinuses and cause a decreased sense of smell?

This would not be expected with oral yeast infection (thrush), and certainly never the only symptom. Many viral infections can do this, however. You need to be retested for COVID-19. Good luck.

Agree with Dr Handsfield, as infectious possibilities are potentially the cause of dysosmia in your case. Agree that a COVID-19 test be included in the evaluation.