For azalea seedlings in peat 2:1 perlite, whatever loose soil you pack on top of the current rootball, that area will eventually all be filled with moss roots and moss. I usually leave 2cm of extra top soil. So every seedling I repotted, after being in there for a year, there was a very clear transition between moss roots and azalea roots. I can pull off the moss part intact and separate it from the azalea.
I do see seeing a slight relationship now between chlorotic cuttings in pots filled with liverworts vs no health issues for actual moss. But for cuttings only, so far. I'd say moss is good to neutral, liverwort is probably bad.
What about growing your actual local moss in a plate of kanuma soil? And then seeding your top layer of kanuma just above the roots of the satsuki with that actual moss? Avoids the whole dried sphagnum need.
If you keep the kanuma in a shaded area and keep it wet, stuff like moss will start to grow there eventually. But you could even find your local favourite moss and grow it deliberately.
If you have nebari like that, having moss grow in between seems like a good idea. Rather than covering it all up with additional kanuma or dried sphaghum.
I got some satsuki that had sphagnum top dressing. But eventually I just pull it out anyway. Keeping it there to keep roots moist and cool 1 to 2 months after a big repot could be nice. But it is not needed I feel in the Swedish climate.
If you get a true heat wave, you could apply and remove it for just those few days, if there is no actual live moss there yet.
I still wonder about the actual full effect of moss, especially in the cloudy wet parts of the year. Surely, it helps the top roots stay moist and cool during hot dry weather, which is very good. But does it keep things too wet in during the colder rainy parts of the year? Does it reduce aeration? Does it improve the overall microbiome quality? Does it help the azalea roots?
Could you put an azalea bonsai in harder substrate and never repot it and keep the moss untoched on top for 10+ years? Robinson style?
I like to pull out moss when I see too much. Often, I'd pull it out and put it back upside down. But I have no idea if my urge to do so is actually bad. But moss always grows there anyway, so no reason to start a big fight with it. Plus it looks good. Just wondering if slowing or replacing the moss every once in a while is good. Like remove most of the moss at the end of the growing season and put in some new fresh kanuma.
I need to dig into the Japanese magazines and find and translate what they say about moss and bonsai health.
From seedling trays covered with sphagum, those rarely seem to fill out with actual live sphagnum.
While the picture of your Nyohozan looks quite dark, it seems very healthy now. Huge improvement considering the burned leaves when you started this thread.
A repot can often act reinvigorating to azalea, and help them grow new roots and new shoots. They like fresh airy soil over compacted decomposed muck.
I'd say, keep doing what you are doing.