As you can see in the first photo the tree looks healthy..the trunk is 30mm across and the tree is about 1.9m but seriously worried about that bark dieback on the lower trunk..it’s more than halfway round..looks like it might snuff it to me, what do you think? Can anything be done? There’s another section similar but not quite as bad 200mm from the bottom and early signs of dieback as shown in the first photo, up the tree
For the most part, it looks to me to have had something kill some areas of the cambium and it is now 'healing' (growing new bark over the damaged area). In your first pic there are some black areas, that is areas of cambium death, that are a lot like the images of pseudomonas syringae on acer palmatum. Mechanical damage can also result in similar affects, but I see no indication of a cut to or tear of the bark that would be consistent with something smacking into the stem, including an errant pruner/knife blade. I would guess that your sango kaku has suffered pseudomonas syringae infections over multiple winters and has recovered, just as it will this time. Copper sulfate fungicide application before next winter may help prevent the next attack. Meanwhile, the cambium is dead in the black area and it will, in time, come to look just like the other places on your tree, where it is recovering from past infection/damage, IMHO.