Ok, assume you have EDTA, what next?

Add Ammonia? The most concentrated you can get is 28% and so that will limit you to the final concentration of Ammonium EDTA, and how many Ammonium ions do you need? NH4EDTA, (NH4)2EDTA, (NH4)3EDTA and (NH4)4EDTA all exist. So, lets assume you want (and get) NH4EDTA, what next. It will be very dilute so you will have to purify it. Next, ow do you react it with Ferric Ion to get pure (NH4)FeEDTA. Note that the amount of Ammonium ion is really essential to balance the charge on the final ion!

This is really a thankless task!

Now, as to the essential need of using Ammonium Ferric EDTA. Well, we made the first bleaches and blixes for C41 and E6 and Ektaprint 3 using Sodium salts and there was no way that we could go to completion in a reasonable time. As we went to more Ammonium ion, the bleaches and bliexes sped up. So consider that EK recommends 6' at 100F (about 40C0 with an essentially all Ammonia bleach, then figure that it might take 12 or more minutes to do it with all Sodium. Then the presence of other cations would fall somewhere in between those extremes. It is not a straight line though.