I think we are seeing a picture her, mag def, zinc, iron, nitrogen! Underfeeding maybe?!
So even if it was mag deficiency mainly, without the right levels of the other ferts its not likely to be cured with just mag. Get your trace elements sorted, think thats the bioessentials and increase the ferts bit by bit maybe with a touch of mag - if it is not present in the bioessentials or your water (test your water ppm to find out)!
I add my seaweed or trace elements once every few waterings as the soil dosent need very much, can be easy to overdo the trace elements a lot of times, one or two applications should be enough for most of flowering.
As always try this first and await the results. Bottom leaves dying or going yellow (as in one pic) whilst the rest of the plant is green and healthy suggest a deficiency. 3 1 2 is a touch low in phosphorous but dont change this yet, try increasing the ferts and trace first.
When i first flushed properly i noticed it left hardly any ferts in the soil, plant started to starve in under a week, now when i flush i add almost a full strength dose of ferts and trace elements back to replace what was loct from the soil. It is easy to get out of balance with flushes and ferts, consider this too. Good luck.