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.

It would help if you had some pictures of the leaves before they fell to rule other things out, but it looks like a blight to me. That spreads by spores entering injured plant tissue. That's why you see the plant producing resin, it's trying to block off the sites where leaves fell so the disease doesn't spread to other areas.

It could definitely be injury from water stress, but could also be temperature stress if you have large fluctuations in anbient temp throughout the day. I kind of wonder if watering by placing the pot in a tub would be more beneficial? Plants will sometimes take up more water that way, but with the mix you have it won't be staying in the substrate long enough to cause rot

I don't know anything about plants but take my virtual support hug. 🫂