I kind of edited my post to clarify. With a long term goal in mind, and the competition being secondary, I would do it like this.
Late August-early September 2020:
Get it out of the nursery pot, remove the bottom 1/5th of the roots if it is really pot-bound and that part is soggy.
Rake open the root ball. Likely, the surface will be covered with roots trying to find a way out of the pot. Open those up. Use a chopstick or something similar. You will tear out some pieces of the root system, but that is fine. It has plenty. It looks like you already raked away the survace of the root ball. Often, there is a lot of leaves, or moss, or the nursery uses mulch.
If you can wash-power blast out some of that old soil. You aren't bare rooting it, but sometimes you can at least clean up some roots by washing them with water, depending on how many roots there are.
Pot it in a terracotta training pot with 50% kanuma, 50% coarse peat/potting soil. I don't why it was to go in a bonsai pot right away. The display is in 2025. You want results in terms of growth for a couple of years.
Then it should be growing a ton come spring 2021. Which is what you want. But at some point you want to go in and prune a tiny bit, to prevent reverse taper, or prevent branches that will be needed for the final design that are 5 shoots from one node. So reduce to two shoots two leaves, kind of pruning. You want to prune as little as possible because you already pruned heavily in 2020 and you will be pruning heavily again in 2022.
Then spring 2022, assuming it has been healthy and growing a lot in 2021, you can cut back aggressively, keeping in mind a final design. And then cut off the lower 2/3rd of the root ball. Then you wash out as much of the soil. Then go in with a chopstick and try to get inbetween the roots as much as possible and remove stuff. Then observe the top of the roots and see which top roots need to be pruned away. And you repot it, maybe now in 100% kanuma and a bonsai pot, with the root flare you like. The roots then will look like the picture in
Woocash's thread. Not a lot of roots, but also not a lot of leaves, so that's fine.
Then come spring 2025, it probably need another repot. You could then decide on your final and smaller bonsai pot. But since you want to take your display picture, you won't do a lot of root pruning. Or you can repot after you take your display picture. Or wait a year if the kanuma and drainage/aeration still looks fine and it still has room to grow in that pot.
But after the competition, nebari-wise, the roots you have selected as your nebari won't really fatten up unless you allow the tree to grow out like crazy. So you would put it in an larger size training pot once more, to develop it further. Those top roots won't fatten up unless they are supporting a large root system, significantly larger than the roots needed to keep it alive in a bonsai pot. The tree will also create new roots that are not part of your nebari flare. And those will get fatter as well. So after 2 to 3 years, you have to go in and prune those downward-facing roots away. The stronger you want the nebari to be, the more of these cycles you need.
If you keep it in a bonsai pot, the change you will see is more that it will look older. But your trunk and nebari won't get significantly fatter/stronger as there is always a limit to how much it can grow, dictated by the size of the pot. Of course, you can go for a slightly larger pot every repot. But this is still a slower process compared to how azalea bonsai are normally grown in Japan.
Those impressive azalea bonsai you see, they are grown in raised beds of kanuma covered by a polytunnel. Inside those, they likely have twice the growing season as we in the UK-western Europe. And they put them there, starting as a wired up whip, for 10-25 years, depending on the desired result (meika tree or actual bonsai). So when you are thinking bonsai and nebari, and thinking about your plant right now, that's something to keep in mind.
