Hello! I am a new user on the forum. Having found tons of useful tips for all kinds of houseplant issues here, I am hoping that some of you can help me with my plant problem: my droopy ficus tree.

I got this tree on sale last summer from the supermarket. It looked great and stayed perky. It even unwrapped 2 new leaves that summer. Then I noticed some kind of tiny centipedes that lived in the soil. I kept an eye on the plant and they did not seem to be harming the plant, but eventually I was worried enough about pests spreading to my other plants that I repotted the ficus sometime in September 2020. I removed all the old soil and washed the roots in warm water several times to remove as much of the old soil (and any bugs) as possible. Of course, I tried my best to be gentle but I did lose some fine roots in the process. The roors looked a pale creamy-white color and I saw no rot. It was also a bit chilly outside while I was repotting (I'd guess 15C/60F) - in retrospect, I wonder if this damaged the plant.

I oven-baked the fresh potting soil (an all-purpose houseplant soil recommended to me at my local garden center) hoping to kill any gnat larvae/pests, let it cool completely then mixed with some "granulate" growing media as I could not find perlite, hoping to improve the drainage as the regular houseplant soils have been too dense to properly drain in my past experience.

I put the plant back in its old spot after repotting. I can't remember if I watered it immediately or after a few days. Within days, the plant started to get droopy leaves. I let it dry out almost entirely before watering again, as I read that after losing the fine roots the plant can't absorb water as well anymore and it's easier to overwater. No effect. I tried watering a bit more frequently. No effect. I went back to watering when the soil is dry when I stick my finger into it a few inches. No effect. The bottom leaves are turning a bit yellow/brown and seem to be hanging on by a thread. As I was moving the plant now to take the photos, a relatively healthy-looking bottom leaf fell off very easily. I have not fertilized the plant since repotting nor added any plant food/anti-pest products to it. The youngest leaf at the top was still standing until this week, when it also started to droop and made me very worried.

I am including photos of the plant, the fallen leaf, and the soil (with cinnamon sprinkled on top to prevent fungus growth - I've been battling gnats on and off). I live in a pretty low-light apartment that only gets a few hours of direct sun in the summer, almost none now in the cloudy rainy season. Almost all my plants take very long to dry out, unless they're in small pots, so I generally water them every 2-4 weeks, as needed (always poking my finger in the soil to check or lifting out the nursery pots to feel the weight, or both). I know this sounds very rare, but after experimenting this seems to be the best method for most of my plants, otherwise the soil stays wet and gets moldy.

This particular ficus is right up against a west-facing window, normally without the curtain - I drew the curtain just for these photos. The window is shaded outside by a permanent terrace canopy. I have another unshaded west window that I could try to move the plant to, but would have to position it further away from that window (~1-2m) and worry that it would be too dark. All 3 of my windows face west and are on the lower-light end as I'm on the ground floor and surrounded by trees/other buildings so I see very little sky from the windows.

Please share your advice to help me save this ficus!

P.S. I recently found thrips on my monsteras but could not find a single bug on this ficus tree, so if it is pest-related, it's completely invisible.

UPDATE Nov. 14, 2020:

I got worried enough to try repotting this plant. However, when I finally got it out of the pot, most of the roots stayed in the wet soil. The parts where roots broke off were mushy, so I guess the plant has been suffering from root rot. I washed the roots in tepid water, cut off any rotten roots I could see, and now the plant is left with just a small amount of roots, as seen in the photos.

What to do with it now? Can it recover as is or should I chop off half of the tree and try to root it separately? I saw people online saying that it helps to prune plants to give them the best chance of re-growing good roots, but I don't want to cut leaves or stem until I know this will help and not hurt the plant.

When I pot it again, I plan to use this terra cotta plant that is a bit smaller in volume as it's narrower than the original nursery pot (the plastic one). However, I would also like advice on how to best clean this terra cotta pot as it lived outside and then in the basement for a long time and, although I washed it with dish soap after the last plant, still has some residue (as you can see in the photo) and I worry that this can transfer fungus/spores to this ficus.

Any advice on how to save this plant would be appreciated!