First off the 5:1:1 mix is too low in K and P. It needs to be in a ratio of 3:1:2 to 3:1:3.

Potassium is the major catalyst for all sorts of reactions. It is needed everywhere to do all sorts of things but it is never bound up. Without it the plant shuts down.

Dropping lower leaves is often a sign of Phosphate deficiency.

You’ll also need to make sure there is enough micronutrients like magnesium, iron, etc for the plant. It’s easy to miss these

Citrus plants are also very water sensitive. To wet and they get rot. Too dry and they drop their leaves.

To me it looks like they are too dry and not enough nutrients.