Wiz, I have the same light and I have the same issue from time to time.
First, get the light UP to 36" above the canopy. Dimmer or not, at least 36" above the canopy. Let them grow into the light, believe me, that light has plenty of power.
These were never closer than 36" grown under my SF-2000:
View attachment 4779078
Second, what is your humidity in the tent? Lately, I've been dealing with low humidity causing nute burn due to the colder weather. What I have found is humidity below 40% in the tent causes the plant to use more water from the roots to compensate for taking in less water thru the leaves, this causes an imbalance as the plant can't use the all the nutrients in the extra water being used to compensate for low humidity.
More here:
https://www.cannabisbusinesstimes.com/article/moisture-matters/
My issue primarily looks like a cal-mag deficiency, even though I am using cal-mag I still have the issue unless the humidity is above 40%. This has been happening since these seedlings sprouted back on 11/10/2020, right out of the dirt they were having issues until I got a humidifier and put it right in the tent. This is my first winter growing in this house, and my first winter growing in over 5 years, so I am just getting dialed in myself.
As for the soil, I only use Fox Farms so I have no comment on that except FF has done fine for me.