I'd add DTPA to the CMS+B mix, say 1:4 ratio DTPA Fe(sequestrene 330 etc). Also, merely having hard GH karst limestone tap well water etc....does not imply that you are getting enough Mg...........I've never found any tap to date that was all Mg and no/little Ca, but have seen a lot that had low/absent Mg, and lots of Ca.
So add a tablesppon of MgSO4 for each teaspoon of CMS+B.
Dose this at least 3-4x a week. Also, well water from karst systems is loaded with CO2, so......large water changes are particularly useful.
I'd add just the K2SO4 after the water change.