My results are getting better. I am pretty sure it is not loose belts. The are quite tight.
I have slowed it down a bit to 1800mm/min and that seems to have eliminated the ringing. I also printed and added another mount bracket the the laser holder. I am using your laser holder vicious. With just one it seems to be a little wobbly and prone to vibrating. I am trying another pass at 2400mm/min to see what it looks like and hopefully increase the depth of colour.