How can I go about reducing a High Mandibular Gonial Angle and a short chin? (Photo)

You are correct in that you lack vertical length to our jaw angles. Without front view pictures no comment can be made as to whether they need width as well. Computer imaging would help you determine if any changes need to be done to the chin once the jaw angles are lengthened. I suspect that you would not.

Your viewpoint appears correct, in that angle jawline implants placed in consideration of the width and height of the 'ramus/angle' would provide that balance. (see website examples). But images from the front would help the assessment. Xrays also determine the 'amount' of change desirable