You really done good, although that mix in 5) may have been a waste of time, energy, and money depending on what nutrients your soil already has.

Still stuck in chemical world?

The current guidelines for planting trees is to avoid any fertilizer of any kind in the planting hole.

If you think you really must fertilize, wait until the new leaves are fully expanded during the next season. Then apply nitrogen (use your preferred product) within the dripline and water in.

In many soils, supplemental fertilizer isn't needed for trees. So it depends upon what your soil is and what it contains. Only a professional lab test can tell you that.

Not really. That only applies to chemical fertilizer. Totally different. Trees will use them when they need it. Nothing are wasted. It's called nutrient cycling.

And yes, there are people including Dr. Carl Whitcomb recommending that they be fertilized when they are planted although he recommended slow release chemical fertilizer.

They are beneficial for maximizing growth... The nutrients in untouched forest floor is much better than typical lawn. Think about it....

Dr. Whitcomb strongly advises that any fertilization should be done only according to soil test results. And I'm pretty sure (not positive) that his observations about fertilization apply to the transplantation of container grown and not field dug (B & B) trees.

If a good, reliable soil test indicated a deficiency of nutrients then maybe what you spread around that tree will do some good, but if your soil has what it needs what you spread around is not needed and was a wastte of time, energy, and money because it was not needed.
There are lots of people, on the web, that follow the guidelines of 20 or so years ago that newly planted trees need "fertilizer". Fertilizing newly planted trees can induce them to grow when you do not want them to and if fall planted that could mean not winter hardy growth, but along with the stress of this planting, fertilizers also cause more stress further weakening the tree.

The Soil Conservancy here sells seedling trees and shrubs to fight erosion and so on. This is high desert, 13" annual moisture is normal, but we''re now in an extended drought. They have run extensive tests on what works the best. They recommend tilling the soil, planting the seedling with a very slow (like 2 years) release fertilizer, putting a 4 to 6 foot ring of woven black polypropylene weed barrier, and then a 4 foot high protective, 6" diameter translucent 'mini-greenhouse' staked around the tree.

The seedlings sell for peanuts. By the time you put down the weed barrier, add the fertilizer and the mini-greenhouse, it increases the cost significantly. I was skeptical at first. 10 years later, I am now convinced they're right.