Leggy growth here is a symptom, not a cause. The neglect has caused the tree to get leggy.
Sparse foliage is a symptom. It is indicative of a plant not having grown strongly for a longer period of time
If you cut a strong tree, you get backbudding in dozens of places on each branch.
If you cut back a weak tree you get a few buds on most branches. Some branches will just dry out and die
You can get a leggy tree with lush foliage, just at the end of the branches. But growing with heaps and bounds. These do backbud a little by themselves. If you there trim, you get lush backbudding and strong recovery and you will have a full tree fast.
As said, your tree should be lush with young growth. If in my summer my ficus grows a new leaf on every branchti[p every 4-6 days, you should be getting that in optimal climate too. I do not see any of that.
Fertilize, water. Get it used to full sun. Let it do its thing for a few months. Watch it start to pick up steam. Then trim.