At the time of purchase, the tree was in a 14" pot and was replanted in an 18" fiberglass/cement pot. It's placed in front of a east x SE facing window that gets plenty of light especially in the mornings and early afternoon. I watered the tree about once per week (ensuring dryness in the first few inches of soil first) with about 3.5 L of water each time. On 1 occasion, I used more water and noticed it coming through the drainage hole so I scaled back down to 3.5 L.

It seemed to be doing well at first, with plenty of fruit, including new ones growing, and dense foliage. I began to notice significant leaf drop (~10 leaves daily) after about a month, and the remaining leaves seemed to be very loosely attached (i.e. would very easily come off with any touching). I attributed to insufficient light as the days got shorter. I purchased a 4' grow light to supplement it, which seemed to stave further drop.

After several weeks of doing better, I noticed even worse leaf drop, with the tree presently only having 1/4 - 1/3 of the leaves it originally did. The fruit also began to turn orange en masse, which was not the case before. I'd also noticed the drop seems to occur most after watering. In doing some research, I learned about root rot and began to worry that the 1 drainage hole at the center of the pot was insufficient, so I drilled some more around the pot to improve draining. (NB: When I did lift the pot to do so, I noticed the bottom of the soil through the central draining hole was still wet, even a week after last watering). Knowing that it takes a lot of energy to sustain fruit, I've also recently clipped off tons of fruit that had turned orange.

After doing further research, I began to think I was underwatering the tree (leaf drop while still green, curling leaves from side to side), and that's when I decided to seek help to determine what's actually going on. I'm attaching some photos of what the tree presently looks like. I'd really appreciate any help as to what you think might be happening, and what I can do about it. Thank you!