Whats the difference between a mild/starting aspiration pneumonia and a severe one. Is there less lung damage if caught and treated early? Thanks!

Yes. Identify the cause? Stroke, swallowing abnormality, regurgitation, hiatal hernia , esophageal diverticulum, obstruction etc. Fix the problem. Left alone, recurrent pneumonia, lung abscess sepsis are serious.
You are correct. Mild bacterialpneumonia generally means there is less lung involved and the person with the infection "stable" with normal blood pressure, heart rate and oxygen levels. Generally the more lung involved, the older (or younger if infant) the person or the more chronic health problems some one has the more severe the consequences of pneumonia. The earlier you treat a bacterial infection, the better.