Pre-covid (and for many years prior) my cbc results were always normal, and during acute covid my eosinophils were 0.09 (x e9/l), but now at 5 weeks after symptoms started it's up to 0.5 (mild eosinophilia?). what could cause this?

There is nothing to concern about mild eosinophilia. Allergic reaction with pollen in the Spring or a new medication can make that change. If you have rash or any other feature of allergy such as sneezing, itching eyes or asthmatic symptoms then a causative allergen should be identified and eosinophilia gives us a hint. Steroids such as Prednisone or Decadron can lower eosinophil count to minimum