Andrew,

It looks like you have been doing a good job at shaping your young tree. You have a nice strong leader. You could now remove the lower branches but you want to make sure that you don’t remove more than 20% of the leaf canopy in any one year. It’s easier to mow around the tree once the canopy is lifted above 6 or 7 feet. You could do this now or wait another year because all of that leaf matter will be photosynthesizing and feeding the trunk.

With maples you probably want to wait to prune until it has totally leafed out or you will experience some ‘bleeding’ at the cuts. This won’t hurt the tree but can be messy. Below are a couple of good info sheets on pruning that I think will help you. 

https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs#pruning-and-training-young-trees-and-shrubs-1335961

https://extension.umn.edu/news/tree-pruning-0#:~:text=One%20option%20with%20these%20types,quarter%20of%20the%20live%20foliage.