Also if the Japanese maples form a circle around the tree, some of them will always get full sun unless they are eventually right under the tree canopy. In that latter case there will also be competition for water and nutrients.
A raised bed will not keep the roots separate but if you're going to do that do it before the 'Autumn Blaze' sends its roots out to where the raised bed would be. Maples don't like the soil depth to change over top of their existing roots (which will extend out past the diameter of the tree's canopy).
