How could I improve my negative canthal tilt? (Photo)

To create more almond shaped eyes, it requires elevation of the lower eyelid plus canthoplasty, not just canthoplasty. See following link and video.

However, you have upper eyelid ptosis and both of these issues need to be addressed.  Do not assume the reconstructive oculoplastic surgeons are the same as surgeons who do aesthetic reconstructive surgery.  Multiple opinions are helpful and in my opinion you will need to travel to get the best possible result.

With your negative canthal tilt and upper eyelid ptosis, you should ideally have a combined lateral canthopasty with upper eyelid ptosis repair. That will create the best overall eye appearance change.