basically it means that you have axonal degeneration or damage to multiple sensory and motor nerves. the axon is the part of the nerve that carries the electrical signals. The velocity and amplitude of those signals are slower and less because the axons are damaged. over time the covering or insulation of those axons, the myelin sheath will also start to deteriorate as they are affected by the deterioration of the axons they cover.
I have the same diagnosis. My progression has been slow and steady over the past 12 years. presently i am numb above both knees and to a leeser degree in my hands and lower arms. i just had emg/ncs's on my feet, legs and lower back today and on my hands and arms 4 weeks ago. There will be no happy ending to this story for me. My cause was exposure to toxins.
Your progression sounds slow and steady also to this point.

What are/were the initial symptoms of Axonal Sensory Motor PM?

for me numbness in both big toes and foot pain.