If you don’t have a growlog please answer these questions.
What strain are you growing? Is it Auto, Fem or Reg?
Are you growing Indoor or outdoor?
Are you using nutrients? Which ones? How much? Mac1 x Candyland/ Outdoor Fem /Elm Super Bloom every other water cycle
I don’t see any magnesium deficiency, looks more like some potassium deficiency. Also 100% agree trimming your plant will make it grow much better and produce way more
The white veins and purple stems, I don’t see the edges burning like a potassium deficiency would do to the plant. Possibly a lack in a lot of nutrients or could be a pot issue? Maybe rootbound.
I see edges burning at the bottom of the plant, remember potassium deficiency always starts at the bottom! It also causes weak and purple stems. The plant has enough magnesium, adding calmag would not help at all. For magnesium deficiency you always want to look for interveinal chlorosis
I would recommend using a fertilizer with good k content, specially during flower. Adjust your PH to 6.5 so your plant can better absorb it. Pruning the lower parts of the plant will also make it consume less potassium and will help a lot.
Intervenial chrorosis and white veins, personally a dose of some 0-4-4 and Calmag together would help. I seem to find plants almost can’t get enough calmag, give it more Everytime u feed it, it’ll love it.