The best method to learn Morse is the method that works for you. I've tried Koch, Farnsworth and others. They have their strengths and weaknesses. The key is practice, practice and practice. Some people seem to have a natural aptitude - I don't. The best operators I know hear whole words not individual characters. They copy it in their heads. I passed 13 wpm and worked up to about 18 wpm. I never got to 20, but I stopped trying. I had an Advanced class license for years. A friend of mine said, "Don't ever upgrade, that way everyone will know you passed the code."
As for hearing impairment, most code practice software and modern radios allow you to adjust the pitch of the CW note. This may help depending on your degree of hearing loss. You can also convert pulses to flashing lights.
One of the older methods of learning code involved random 5 letter groups called cipher groups. When I first got interested in ham radio that's the way it was taught. The advantage is that you can't anticipate the next letter. The problem is that you get used to having a space every five characters. Not good for real language. The cipher groups were a real world method of sending encrypted military traffic and that's how they originated.