Above the pH levels you listed, they do start breaking down at a rate dependent on pH.
For long-term storage in a bottle, Fe EDTA is therefore best kept at 6.0 pH or below.
Dosing every day or every other day, the breakdown rate for Fe EDTA is insignificant at 6.5 pH. Probably for weekly dosing too.
At 8.2 pH is another story - I had to do daily dosing of Fe EDTA at 3X recommended amount to prevent iron deficiency. Switching to DTPA helped greatly here.