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Pruning is actually not recommended in younger plants. Allow it to grow and flower at will. Planting several together can make the appearance more full.
As the plant ages you may began to see less flowering, so this is when you can prune it down; early in spring.

https://www.gardeningknowhow.com/ornamental/flowers/popcorn-cassia/popcorn-cassia-information.htm

It does appear to be a cassia. I think that your identification is as close to accurate as you can get without knowing for sure which cultivar that it is. You can try taking the photo to your local extension service. They will be able to help you a little more. This link will help you to find the closest one to you: https://www.gardeningknowhow.com/extension-search/

Sometimes, this can be of no fault to the gardener. Soil can be contaminated before planting. There are many things that you can do to combat soil pathogens, but I prefer a simple dusting of wettable sulfur and dolomitic lime.

This will treat the illness, but the dying leaves will not recover. They will fall off eventually. New growth will be unaffected and you will know the treatment is working.

This collection of articles will give you an idea of how to care for these: https://www.gardeningknowhow.com/ornamental/trees/cassia

Typically removing such a small tree will pose very little harm to it. There will be some things that you will need to consider. This article will get you started: https://www.gardeningknowhow.com/ornamental/trees/tgen/transplanting-trees.htm

You can prune these back pretty easily. It is best to wait until spring though. This will give it the best chance of fighting off potential infection from trimming. Here is an article for more information: https://www.gardeningknowhow.com/ornamental/trees/cassia/cassia-tree-pruning.htm

I'm in Bangkok and my Cassia fistula (planted in the ground earlier this year) grew like crazy, almost 3 meters during the rainy season. But it is slowing down now with with cooler weather and no rains.

In a pot there is more possibility for depressed growth due to water and nutrient issues.

A good rich potting soil, water and fertilization are the first considerations. If you think the soil is as good as you can get, then the next issue is water.

Adequate water is of utmost importance, consider using a soil moisture meter to take the guesswork out of when to water.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

Fertilize at least spring and fall with a slow release natural fertilizer like this:
https://www.domyown.com/dr-earth-organic-all-purpose-fertilizer-p-9485.html

Consider re-potting with a nutrient rich potting soil like this:
https://www.domyown.com/dr-earth-home-grown-organic-potting-soil-p-9497.html