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Generally, you don't fertilize them. They have a need for high quality compost and soil, so with the correct soil they have enough food to last until each repotting, which will be each year. Repotting will need to be done in a container that is about an inch or two larger on all sides. Any larger than that with each transplant will likely introduce rot, since it will not dry out as quickly.

Here is an article for more information:

https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm

Drooping can be normal, but I suspect that the bacteria infection has something to do with it. I notice the lesions on the trunk, which can cause leaves to droop when near the petiol.

I would recommend Fungicides. They contain the bactericides necessary to kill off the infection. Anything Copper-based will be appropriate.

Here is an article that will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

You should see roots growing in about a month. Here's an article for you: https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm

These trees are very susceptible to overwatering. When it is dry and you feel like it needs water... Keep waiting. In fact, wait until it starts to droop pretty drastically before watering.

They do not tolerate wet soils, at all.

You will need to correct watering habits, and more importantly, treat with a fungicide.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm

Fungus gnats will eat fungus, rather than cause it. They don't touch living plant material, and prefer their food to be rotting, already.

Their presence indicates that the soil may have been too moist, causing rot. The gnats will be attracted to the rotting roots.

It doesn't seem like it but these will be considered beneficial insects. They have their limitations, and will not be able to clear a fungal infection, though. They will only be an indicator of an issue.

Make sure that you treat the plant with a fungicide, and make sure that the soil has a chance to dry, thoroughly, down to about 2 or 3 inches between watering.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Fiddle Leaf Figs will be very sensitive to overwatering, and are usually killed by infection before they reach maturity. Make sure to let the soil dry out some between waterings, and don't let it remain wet and it will have a better chance at survival.

here is an article that will help:

https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm

These are very slow growing. Anything that you do to encourage more growth will likely slow it down, so I wouldn't do much outside of watering it properly. They are easily overwatered and are really bad about contracting infections when they do get a little too much.

Don't bury the stump until your next repotting. This will be a few years since it is in such a large container, now.

Here is an article that will help you to care for the slow growing tree:

https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm