The only thing I would add is this can require a lot of gravel fill. I do something similar, but use the concrete forms (the oversized cardboard toilet rolls) with the post in it with gravel fill, then fill in around it with whatever soil/clay came out of the hole.
Unless you are digging it by hand, which is a massive amount of work. I''ve done that before as well and it sucked when I was in my 20s, let alone in my 40s. Usually the augers on the tractors etc are oversized specifically so you can locate the concrete form properly then backfill around it before setting the post and pouring concrete. But even then used to have some gravel in the bottom to get the post roughly level before the pour, and make sure there is drainage at the bottom.
