Slower growth sometimes is better, you get the same result basically, just may take a little longer.
Salts could be the issue, but the mag issue isn’t driven by over feeding here, it’s
A, there is just zero in the medium,
b, and this is 99% of led users,
The intensity is causing your plant to pull it all up. Causing the tailspin.
I’m never in a rush to finish a grow either, so I respect the fact the slowed growth is an issue, but, overall plant health and proper uptake, super important too, sometimes an extra week and the end of the world.


Hope you get it sorted.
Oh, and how long were you leaving the light throttled back when you did turn it down? And did you raise the light too? No? Because turning it down and dropping/raising it some in concert with that may help or vice versa depending…….thats my 2 copper.