This is a 5 year old Fukian Tea which, until 8/2022 was extremely healthy. In August I left this tree with a friend to care for while I was away from home for a month. Upon returning, the tree had hardly been watered and was near death. Over the course of the next four months, I have removed dead branches from the tree along with portions of the root system which were killed (these roots has become so dry and brittle, just touching them would break them. They were long gone).
Prior to this incident, this tree lived in a 3'x5' grow house (a "Medium Bamboo LED Grow Light Garden" from Gardeners Supply which I have modified). This grow house contains 3 full spectrum LED arrays, mylar reflective walls, air circulation, and (although a manual process) temperature and humidity control. In this configuration, there are approximately 35,000 lumen at canopy height. Although this might seem high, this plant flourished and never showed any signs of stress in leaf shape, texture or coloring. The tree resides in a training pot and the substrate consists of equal parts lava rock, limestone pearock, calcined clay and just a small amount of pinebark to retain a bit of moisture and some nutrient. A bonsai liquid fertilizer is used once every 6 weeks, as this tree requires significant water in its housing conditions and the substrate tends to be washed out rather quickly. Until August of last year, this process yielded a very nice tree.
At this point, all growth seems to stalled. The leaves you see on the tree are the ones that have been present for some months now, with little to no new growth in any area. Many of the branches on this tree died and were removed. In doing so, the tree has lost nearly all growth on one of its sides. I imagine this is a result of damage to the root structure, which also required surgery, as I mentioned in the beginning. I have kept this tree in its original grow house, as that is the environment it did the best in for some years, though I have reduced the light power by 33% and shifted to a 12 hour on/off schedule to reduce stress. Though the leaves and flowers do appear healthy at this time, they are few and smaller in size than they once were by at least 50%.
By looking at the picture I have attached, I am wondering if anyone feels this tree is savable? I would certainly like to, it was a gift from someone I no longer have, so it would be nice to be able to rehab this properly. I am unsure if the living branches that remain will ever produce leaves again, perhaps someone knows. If anyone has any advice, I would be grateful to read it. Though I have tried to learn this hobby the best I am able to, I am certainly (obviously) not an expert and I admit fully that I need some help here.

P.s. Please ignore the wiring. It was done when there were many branches left on the tree, and with the intention of restoring balance over time to the side which lost so much of its mass. I understand it looks like a nightmare. I know.

How long have ypu had the tree?

4 years and some change.

I would suggest to do excactly the same things ypu did during the 3,5 years it was healthy. That is a really long time to keep a fukien healthy.

Doing root work and pruning an unhealthy tree does not help it. But that is damage already done and cannot be reversed.

Ohh, yes. Do not fertilize until the tree is back to full health. On unhealthy trees the fertilizer risk to build up to dangerous levels. Never fertilize an unhealthy tree and never fertilize a newly repotted tree.

Good luck! Please let us know how things progress.