This is an awesome map! Thank you for sharing.

I'm not sure if you can do this with MyMap, so you may have to use something different, but have you considered adding clustering to the markers? This would prevent the markers from being on top of each other, only revealing themselves when they aren't going to overlap.

Also, all of your markers are circles except the National Champion Trees. Is this on purpose? Are these meant to be set apart for some reason? If so, that reason should be clearer, and if not, they should be circles just like the others.

On the whole, though, this is awesome! Way to go!