There is not rule about doing 50% weekly water change, it's just a starting point. I do monthly on a few tanks, others get them 2x a week. I just modify and adjust according to basic stuff: eg........I have mostly ferns and Anubias, and a high fish load, low light, high current. I'll typically do 60-80%, if I bother to change, I tend to go big. Does not take much difference in labor and no more motivation(the real issue).
If things improve without a water change, then perhaps the plants/sediment , wood etc is removing some of the KH? Sometimes adding new water adds a lot more CO2, we often see nicer growth the 1-2 days after a water change.
This can lead to good CO2 1 day a week etc, then poor CO2 the rest of the week. This is often enough to get folks through.
So you could do this: water changes 2-3x a week large %. Then try as suggested one water change once every 2 weeks. You might consider a different Fe chelator.
I had a lot of L acurata which is somewhat similar to L brevipes in real hard tap........it did great, but CO2 would cause issues, once that was adjusted correctly, then things went great.
The other possible issue might be something negative in the tap.
RO/Carbon filtering prior might resolve that question.
EI is useful as a standard non limiting reference, however, it is possible to exceed the non limiting values. In general, it is used to rule things out, not show cause. To find cause, you must rule out the other possible factors, of which there are MANY.
CO2 would be my suspect if I where you. I also would not change the water much and then watch closely after I did them. If 2 weeks works, stick with that for now, but watch closely, then move to 3-4 week ranges SLOWLY.