When fertilizing with Miracid is not an option (after repotting or during dormancy), I' ve been using Kanuma as the upper layer. It works.

I use neat Kanuma only for all my Satsukis, I would never use anything else.

Yes I also use Kanuma 100% for Azaleas, I was convinced once to use a super duper mix from a nursery, and I'm counting down days to repot again to Kanuma the little tree.

I am thinking about next spring when I put all the azaleas I dug up in pots or do something like that. I was thinking akadama, peat moss, lava rock, pumice roughly equal. This makes me think not, just kanuma.

Akadama breaks down after a couple of years, depending on freezing/thawing. How long would kanuma last, particularly 100%? What other considerations are there? Would I stop using acidic fertilizer and use the same as the other trees?

Akadama is neutral to alkaline pH whilst Kanuma is much lower pH that is why I use it as do all Japanese growers.

Whatever mixture you are planning to do just put Kanuma instead of Akadama, Azaleas need acidic soil to develop correctly, and yes you will need to repot every 2 years as Kanuma will deteriorate after that period of time.