I think you’ve figured out your own problem, the extremely bent wrist will cause a lot of tension and discomfort in your left hand because you’re compressing all the tendons, ligaments and muscles in the palm side of your hand.
There are a few things you can try to make yourself straighten that wrist. Keep in mind it has to have some bend, not be completely straight. The main one is to angle the guitar so the headstock and neck are a little higher. That way you’re not reaching down and then having to bend the wrist back up to reach the low notes on the fingerboard. You can also try moving your elbow back a bit. If your elbow is too far forward in relation to the neck your wrist will naturally bend more.
Another couple of things you can try for more comfort is first, if your thumb is in the middle of your 4 fingers try moving it a little more towards your first finger. This should release a little tension in your palm. Also, try not to squeeze with the thumb. The last thing I would suggest is leave your middle finger straight, not tucked down towards the lower strings. Since it’s not playing a note it’s better to have it in a more relaxed position. Hope this helps.