I also have 2 interesting updates:
1.) Formation of mesc fumarate from toluene is not desirable: fumaric acid is totally insoluble in toluene and so also the salt will precipitate so fast, that it only forms really voluminous clouds, which dont crystallize nicely. It is so fine that you cannot even decant it. Though it is mesc fumarate, because the mass is more than just the initial fumaric acid. Still maybe not the best route.
2.) mesc benzoate working too! I feel that concentration is just the issue. It seems way more soluble than it would make sense. A regular extraction might have 500 ml on 100 g cactus. So you would get like 2 g of mesc benzoate in 500 ml.
But I now got a really nice precipitation from 100 mg mesc benzoate in 2 ml.
That means you need to concentrate the toluene a lot and then throw your benzoic acid inside. Still you can maybe just go with 2 g instead of 5 g, because excess is not needed (can anyways form only 1 salt type).
From all the few mesc salts I got now (citrate / fumarate / benzoate) it looks by far the most beautiful. Benzoic acid forms really sharp long needles when re-x'ed. So I was initially thinking if it could be just mostly benzoic acid. But there is an easy check:
Win! But what is it? Benzoic acid? maybe.
Just take a small spoon sample of your material and throw it into water. mesc benzoate will dissolve if you crush it fine enough, while benzoic acid is practically insolube. Just use super tiny amounts. Like 2-3 mg salt + 5 droplets of water. Only the mesc benzoate will dissolve. If everything dissolved you got the salt.
So for benzoate I feel it just has to be quite concentrated, but still above benzoic acid crashing out. But I feel solubility of that is super high anyways.
Actually now when I check your toluene merkin you drop it from 600 ml. That is actually the opposite of highly concentrated. So I am not sure if my theory is right? But it would at least explain why all the others and me did not get any crystals upon initial addition of benzoic acid to a big amount of toluene ...
Still I have no possibility to check overall purity. So a true verification would be cool. I will send it to a friend who can analyze it!
Now the last thing I still hope I can do with my remaining few mg is to try if I can drop it from limonene as benzoate. Second important thing would be how low does it need to be concentrated. Is the concentration after extraction enough? That would be important, otherwise it's not feasible ... because you cant just evaporate limonene like you do with toluene. And back-and-forth extraction into a smaller amount of limonene would be laborious and probably wasteful, so this route would then be not interesting I guess.