I had 2 installations of QGIS. One was through OSGeo4W, while the other one was installed independently.

I too  faced the same qgis import error issue. This is what I did.

**Just *TWO* changes:**

1. **PYTHONPATH** = 
   C:\OSGeo4W\apps\qgis\python;
   C:\OSGeo4W\apps\qgis\python\plugins;
   C:\OSGeo4W\apps\Python39\Lib\site-packages

2. **Path**(in system variables):
   C:\OSGeo4W\apps\qgis\python
*(Add this path and "move down" any other existing path to QGIS)*

Also, make sure that the paths that you require are moved to the top of Path variable list.