A slow left yaw is probably due to your transmitter not outputting 1500Us at center yaw. You can test this by checking out the receiver tab in Betaflight.
Once on the receiver tab, you should see a representation of the model slowly Yawing left.
You will probably also notice that your yaw value at center is some number less than 1500uS.
Although one would think that you could trim this out via your transmitter trims, this is actually incorrect.
The correct procedure would be to edit the mixer subtrims in the radio menu to get the value to output 1500 while the the trims and sticks are center. Do the same for the endpoints to make sure that minimum reads 1000uS and max reads 2000uS.
This should get you flying straight as an arrow.
Let me know if that helps to answer your questions!