Thanks for being brave and sharing those pictures. Most of us have seen similar in our own tanks at some point.
Nothing in the additional info is jumping out at me as the culprit. I see you have a drop checker, and assume you are using a proper 4°KH solution. I see also you've reduced light, which *did* look excessive six months ago or less.
Once you reach this extreme level of algae infestation, correcting an underlying problem typically has little impact on this kind of algae. It seems to be able to modify its own environment to be favorable for continued growth, even beyond the obvious detrimental health impact on plants.
Gotta hit it hard to turn a tank like this around. Wipe out all you can in one pass. If you haven't tried the Excel overdose whole tank treatment yet, by all means do it. It works well most of the time. If you have tried that and it failed, I have an interesting (but experimental) treatment that's much more potent.