Looking at the tree's leaves there is no sign of a nutrient deficiency. Therefore, the problem must be inside of the container. Either watering habits, or soil compaction. When a medium begins to compact, or retains too much water, the root system has less and less oxygen. How long has the tree been potted in the same container? A 5 year old seedling tree should be much taller.

Thanks Jodi - that's super helpful. I moved it into this pot about 1 year ago and it's struggled ever since. You think I should re-pot it to make the soil less tight?

Yes, and when you repot it use the 5-1-1 soil mixture. You can readily find a lot of information on the construction of the 5-1-1 medium on this forum.

If there is leaf deformation with the curling and stunted growth, it could be broad mites. They are too small to detect without a microscope unlike spider mites and much harder to deal with for that reason. If you have ever grown an African violet with distorted flowers.... That is how I got an infestation. It looks like a virus but isn't. I had a bunch of plants that were badly infested - dahlias, bell peppers, torenia - all had stunted growth, leaf curling downwards lengthwise, deformed new growth/buds, stalled buds....

Nicholas you have answered your own question. You have repotted and the plant has been stressed ever since. No question repot with a different pot and different media. Probably poor drainage, not enough oxygen in the roots, and too much fertilizer and the lamp too close. Plastic or ceramic pots don't drain well. Clay pots are much better because they allow air and water to pass through. The leaves look like they are wilting. What is the maximum water retention of your potting mix and how often do you water? What do you water with? Too much or too little water has the same effect. Too low a humidity or too high a VPD has the same effect. Spider mites can also dry out leaves. Your lamp looks too close to the the plant. When you place your hand between the lamp and the leaves can you feel any heat? Citrus are very susceptible to lamp heat. Look for a delux potting soil that does NOT contain any wetting agents. A reputable potting soil should indicate the maximum humidity or the maximum water retention. You want something around 60%. The plant has grown over the 5 years so its requirements for light have also increased. The light intensity for citrus sinensis is 3700 Lux to 70,000 Lux. Using some horticultural mylar film stretched over some canvas picture frames you can reflect more light towards some of the lower leaves. Ideally with one plant you would be better with a grow tent where you can raise the humidity and get better bang for the Lux. The root problem (garden pun) is the pot.