Hello!
Air layer is a good way to get some advanced material, with some thickness already and cool radial roots if you get lucky, and it seems like maple trees are really responsive to this technique...
About your forest, I would have raised them for a few years on their own pots to get some good thickness before having them planted together, they still too slender, however is good to have different experiences, let this one be, see how it goes, and work on some other trees in the meanwhile, if you have some material on your reach, it's a good idea...
Also, if you have a garden, consider training some of your trees on the ground, you can get some good material way faster field growing, do some research about...