PPM Meter is plenty fine for reading nutrient strengths. Not sure what nutrients you are using but I'd recommend getting something like General Hydroponics Flora *ova (site is censoring the word, begins with an N) Grow and Bloom, they are very simple to use. I'd also recommend a bottle of Advanced Nutrients pH Down. It is a stronger pH down than GH's version, meaning you'd need less and the bottle would last longer. Those are the things I'd do to get the plant health back on track.

rose slugs (sawfly larvae) - they look like teeny, green caterpillars and hide on the back side of the leaves

It could just be stressed from repotting, many trees take a turn for the worse after then perk back up after a few weeks. My first Meyer looked like that and I severely over watered it. It was my lack of experience with citrus (it looked sad so I kept watering it) and using a really water retentive soil mix (miracle grow potting soil) that eventually did it in. You totally can successfully grow in potting soil but you have to make sure it dries out between watering's. I can't see the whole size of the tree but in a more water retentive mix you generally will want a smaller pot than usual to combat how much water is being kept near the roots, bigger pot = more wet area for the roots to sit in and if the root system isn't big enough to utilize all the water quickly it will stay wet for prolonged periods of time and potentially rot. I assume you watered just before taking the picture and that is why the soil looks so wet but when you go to water again, stick your finger 2-3 inches into the soil half way between the trunk and the edge of the pot. If it still feels damp hold off, you want it to be almost dry without the tree wilting. It can take some trial and error to figure out how often to water in your area. I assume it will spend most of its life indoors in your zone but when the temps are above 55 degrees bring your tree outside and let it grow there until the temps fall, it will be much happier.

When plant is not getting optimum conditions they are much more susceptible to various problems. My guess hottunia bed was really rough place to live. ... As a result plant got all kinds of bacterial and fungal funk. Might take a bit to recover next year but with TLC should be back. No different than humans with compromised immune system get sick and sicker more than ones with strong immune response.