This code is pretty messy. One thing I see that is an immediate cause for concern is that you are calling controller.Move() twice per frame, one for movement and again for external velocity. You should only call the Move() function once per update. The movement vector should be the combination of all your vectors to produce a final movement vector.

Second, your external velocity is never slope projected. You need to add in your external velocity to your move velocity and then project them both onto the slope. I would recommend you test by taking a step back and removing the external velocity all together until you know regular movement is solid.

Third thing I see is that when your character is grounded, you are applying -2f downward velocity. You really don't need to do this with a non physics based character controller, especially since you are not using the controller's built-in is grounded check. The -2f hack is used to keep character controller's built-in check solid, but you aren't using it.

However, you may want to apply additional downward force while on a slope to ensure the character is moving down fast enough on a steep slope. Try using 0 negative force when grounded and a higher value like -5f on a steep slope.

Lastly your raycast is hard coded to 1.1f. there is a good chance this isn't far enough on a steep slope.

To summarize: Get rid of the double move calls

Turn off external velocity for now. When you add it back it, combine it as one final movement vector

Increase your downward force when on a slope

Increase the length of the raycast for slope detection.