I knew I wanted to do it, and decided why not now. I figured it was growing well so it’d be ok
All of this is going to sound harsh. It is.
But, You said:
"My goal now is to spend more time learning how to keep em healthy and growing"
You're making all of the big mistakes beginners make when collecting trees. They get anxious to move ahead with plans immediately. they make assumptions that aren't based on much more than a hunch.
It's not a great way to go, but well... Impulses right after you dig a tree are usually wrong, unless you've been collecting trees for a long time. "Wanting" is not a real reason to do things. "Why not now?" is probably a great thing to put on the gravestones of dead collected trees.
"It's growing so well" is another mistake.
As has been stated before, some trees (and I've had it happen with extremely vigourous older Cedar elm) push a lot of new growth just after collection, especially older trees like this one. That growth stops about three to five weeks in. Then the tree sits. At this point, beginners will think everything is just fine. It's not, necessarily. That pause means the tree's initial push of growth was fueled not by new roots, but by simple inertia of the tree using reserves.
Sometimes, that pause leads to a withering of the new foliage and then death of that foliage, then death of the tree. I've had it happen. I've learned from long-time collectors such as Zach Smith, that bagging the trunk with plastic can get newly collected trees past this problem.
Collected trees are best left alone for at least the following year, two is better...
By roughly sawing off the top after you let the tree sit, you may have damaged developing roots. Also, ragged chops like that can dry out the trunk left behind. big cuts need to be sealed. Sealing that ragged wound is going to be difficult.
Doing stuff because you feel like it and not because the tree needs it, is a slippery slope to a lot of dead trees.
This is a pretty decent tree to work on. It's worth taking the time and thought to give it (and you) a fighting chance to really develop it into a nice bonsai and not have it wind up on the burn pile...