EDDHA/EDDHMA is perfectly safe for aquarium use.
The only downside is that it will tint the water blood red if you use to much.
You can use DTPA or EDDHA with KH2PO4 in a stock solution, as long as you use some kind of acid (ascorbic, citric,...) to lower the pH in the stock. This way you will have zero precipitation.
EDDHA is mostly 6% iron, and only if you have very good ortho-ortho quality, otherwise it will be less. DTPA has a larger iron % and is usually cheaper.
There is another high grade iron chelate, HBED, which has even better properties as EDDHA/EDDHMA.