first in order to destroy the chelaters.
Bottle 1 uses sodium hydroxide & acetic acid (to destroy the chelators, so that the iron gets unbound & detectable), bottle 2 Phenantrolin and so Ferrous gluconate can be measured. (so this means it is wrong too say, this canīt be measured if many 5$ hobby test kits can do this) By adding bottle 3 (Ascorbin acid), some chelated iron like EDTA can be measured, too. Well, since EDDHA is pretty stable on a very wide PH range, it just seems not too work with those ingredients to destroy such "harder" chelators.
On the other side,
Fertilisers like the Elos series contain lots of very stable chelated Iron and it is hard to believe that there should be no iron test availible for such folks.
Certainly, sending samples to a certified laboratory would do the job, but for doing this you need to have and find any affordable laboratory. And even if you would find one, it will not help much if want to track the iron uptake daily.