Just a few paragraphs into THIS OTHER ARTICLE about Calcium and Magnesium I learned that Magnesium is likely more important to increasing brix levels due to it being used to make chlorophyll which manufactures sugars, then the enzymes responsible to load and move those sugars.
In soil Mg is not a thing. Here's why:
Chlorophyll chemical formula = C55H72O5N4Mg ONLY 1 Mg atom there on the end.
Gonna be plenty in soil for a plant. The Ca to Mg ratio is whats important.
We like to have a 7 to 1 or higher Ca to Mg ratio. The plant will use up the Ca a lot faster, at some point we get an imbalance of too much Mg - that looks VERY close to a Ca deficiency when its actually a Mg toxicity.
I do not add in any extra Mg to my soil mix.
You have the thing about Chlorophyll and Brix correct. That's where the sugar comes from.
You mentioned Enzymes. They are the catalysts for growth and health. Find yourself some good enzymes and you are on your way to higher brix.
I use Malted Barley, malted organic corn and malted buckwheat grind to a fine powder and mix into the soil. The microbes and the plant take care of the rest.
If you think about how wine or beer is made and then look at whats in the malted barley etc, its the enzymes the brew master wants along with the sugar the enzymes help put the yeast (microbes) into high gear.
To get thicker stems, growth hormones help there. Kelp is a great source. It also happens to have every macro and micro nutrient a plant needs to live.
This is for soil, I wouldn't have any idea how to do it with liquid fertilizer.
I depend on the micro-herd to do a lot of work after the worms break down the soil organic matter into soluble form. All I need is water.
Many ways to skin the cat for sure.
For Brix the plant if she's healthy will move the sugars to the locations that she needs them. They are produced in the leaves but they translocate to other areas of the plant as needed for growth and reproduction.
I'm not a Brix expert. If I was testing I would try different parts of the plant at different times of the day and compare my findings. But that's just me.
If you're using leaves for the brix test, remember leaves have a life cycle and will contain more sugar and different times in that cycle. So age of leaf is a thing.
My brain starts to hurt when I over think this stuff.