My Ficus Bonsai has been slowly losing it's leaves over the past months. They would turn yellow/brown and then fall off. One of the pictures is before the leaves all fell off and the next is it now. There are no leaves left, but there sprouted a little shoot in the photo that you can see.What might be causing this and what can I do to help it stay alive?

Ficus can loose leaves on winter, but that doesn't seem to be the case.
I think you had an issue with watering (over or under), I can see the branches tips are dry, but if sprouted, it can be recovering, however if you are on northern hemisphere, it won't have a solid growing burst until spring.

Maybe you should tell what have you been doing with the tree, like positioning, how long you have it, what's your climate... etc..

I live in France if that helps, and I'm concerned that it might not be getting enough light? I tried watering it more and less when the leaves started to fall but didn't seem to change much.

There was an issue with bugs and mold on the soil so it's been repotted twice which I think may have been the main cause, but this was at the beginning of the summer.

Repoting a Ficus on early summer shoudn't be a problem, as they still have time to thrive before winter, unless you removed a certain amout of root mass, and didn't reduce the amount of leaves, that can lead the tree to dehydration, mold and bugs generally is a sign of overwatering... Maybe because it's roots wasn't being able to consumpt all the water on the substrate.

Do you keep your tree outside during summer? Does this window gets some direct sunlight if you open the curtains?

I don't keep it outside in the summer as I live in an apartment in Paris and don't have the option. The window doesn't really get a lot of direct sunlight, but a bit during the day.

As for the repotting, on the second time the trunk seems almost loose in the soil. Like it can wriggle a little freely even still. I didn't trim any of the roots, just washed them and repotted.

Also, thank you for all the help!

The tree should not wiggle in the pot. Every time it do so the fine fender roots gets damaged. And the tree spends all its energy on new ones.