I still think this is iron-related. I don't know how much Seachem recommends for dosing their iron supplement, but I would certainly reduce it from what you have been doing. Some of the stem plants have faint traces of similar, though not the swords that I can see.
Plant nutrients must be balanced; each nutrient is required in a fairly specific proportion to all others. This is one reason i use and recommend comprehensive fertilizers like Flourish or FlorinMulti; they are balanced. As soon as you begin dosing any individual nutrient, you are upsetting that balance.
This can be OK in certain situations if a particular nutrient is lacking from the other usual sources. Example being the hard minerals, calcium primarily, along with magnesium and potassium. IF these are insufficient in the source water (tap or well) an additional dose beyond what is in Flourish can benefit. I need to use Equilibrium because of my very soft tap water, basically zero GH. But I have never dosed iron, at least not since the early 1990's when I did (on the advice of the store) and had problems then.
I wouldn't suggest Equilibrium yet for you, as I don't see evidence of hard mineral deficiency. Rather than use iron on its own, I would experiment with a second weekly dose of Flourish, and after a few weeks if the plants seem to be lacking something, a third. I have done this with good results. The point here is that it is keeping all nutrients in balance. The iron in Flourish should be sufficient, as it is balanced with the other nutrients.
Sorting out specific nutrient issues is not easy; the common deficiency symptoms can apply to more than one nutrient, and finding the one that may be missing a bit is tricky.
Hope this helps. In summary, I would reduce the iron (probably all together) and increase Flourish to twice weekly, full dose each time, 3 days apart.