Oh, the "soil" -- it is currently a random mixture of composted pine bark of various non intentional sizes, gravel, sand, decomposing/composted tree branches screened out of the compost. Random additions of tarface type clay oil absorber from the auto parts store, small amounts pumice gravel mixed into peat/pelite/compost based potting mix. (I know -- I hope dedicated hobbyists are not offended by the randomness and casualness
Lest I sound like somebody who lives inna cave on a mountain top dressed inna loincloth. I started as you have, with soil locally collected (OK saved from the compost bin). It just didn't work. I then bought some Oilz Dri as my inert component, which promptly jelled into goo and killed a crop of Japan maples.
In NH granite was routinely crushed for highway use and a gritty dust was a bi-product. That granite "grit" soil as cheap as $ 6.00 a ton, an' a ton was just (barely) transportable by my van. I built a set of screens ( 1/2", 1/4", 1/8", and window screen 1/16") Basically what ever would not
pass through the 1/2" screen was too big, and therefor gravel. What ever would
pass through 1/16" window screen was too small and was therefor greensand and went to the garden. All the bits in the middle became my inert component for bonsai soil.
One scoop of crushed pine bark mulch (aka soil conditioner), and one scoop of sifted granite crumbs became my basic default for bonsai soil.
Now-a-days I don't live in NH and I'm too lazy to chase for a gravel mill that does use granite (in limestone rich Ohio)... My local feed store does sell "Grani-grit" chicken scratch, in 50 Lb bags for a still ok enough price as as to not excise my cheap gland. This kind of granite grit comes in several sizes buy layer (or turkey) over chick.
Now-a-days I almost never need to use my screens.
IMO Elms and mullberry are great local candidates, only surpassed by a local bloodgood Japan maple (or a larch of you live in the north). IMO I would not buy any trees untill your confidence at meeting the horticultural needs of trees approaches hubris. New guys kill new trees with all the regularity of a metronome. I have this Tshirt...
I have $ 100 worth of pressure treated lumber quietly drying in the garage, which may become my next bench if I can ever get to it.
Boxwood and azalea are often discarded from home remodeling, and can become more developed stock to work on while you wait for tree babies to bulk up.
Ah! Sugar maples I would wait till autumn to collect to pots. I might do the same with your gum. All maples can bleed out if cut too awake in the spring. Here in Ohio I'd collect before April 1st or after June 15; and commit them to a shaded bench this year.
When I was going to really chop the heck out of a maple I'd wait till after leaf fall, and rub dirt onto wounds (use cut paste if your squeemish).