For starters, if you don't use the rear seats frequently, fold them for better visibility. You can also remove the headrests. I also remove the front passenger headrest for better side visibility, both direct, and via my rear view mirror (aftermarket longer and slightly curved to reduce right rear blind spot). Seat all the way up also provides better vision.
You may have already tried this, but getting the mirror as high as possible gets you the best rear view. There are two pivots on the mirror mechanism. First, tilt the mirror all the way down at the pivot connected to the back of the mirror. Next, reach behind the mirror and push up on the bar that is between the window and the mirror all the way until it stops. Now adjust the mirror tilt back up so you have the best view by keeping the attachment bar in the same place, just using the pivot on the back of the mirror, not the pivot on the window. Done!
2018 Kia LX HEV Metal Stream with Advanced Tech