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Straight cuts are fine. Angled cuts are for rooting, since it offers more surface area.

The time of year is important, though. Pruning them at the wrong time will cause disease.

These articles will help: https://www.gardeningknowhow.com/edible/fruits/mango/how-to-trim-a-mango-tree.htm

These articles will help: https://www.gardeningknowhow.com/edible/fruits/guava/guava-tree-pruning.htm

https://www.gardeningknowhow.com/edible/fruits/mango/how-to-trim-a-mango-tree.htm

Try and avoid the fruit, but make sure that your solution is VERY diluted out. Using this during the growing season requires special care. This article will help you to use this correctly:

https://www.gardeningknowhow.com/plant-problems/disease/using-lime-sulfur-in-gardens.htm

An alternative, for when they are actively growing is a soil treatment. Use DOLOMITIC LIME and WETTABLE SULFUR together in the soil and water in.

This basically has a similar effect, but it is being taken up by the roots and can be added at any time.

It can take several years to recover from infection. I would expect, at least, 3 or 4 years before you start to notice things return back to normal. If it still doesn't return to normal, then you may want to continue treating for infection.

Next would come looking at environmental factors. You may only be able to control one of those- How much food it receives.

Too much nitrogen and not enough phosphorus and potassium will cause a lack of flowering.

Here are some articles that will help: https://www.gardeningknowhow.com/edible/fruits/mango/mango-tree-not-producing.htm

https://www.gardeningknowhow.com/edible/fruits/mango/how-to-trim-a-mango-tree.htm

https://www.gardeningknowhow.com/edible/fruits/mango/growing-mango-trees.htm

https://www.gardeningknowhow.com/edible/fruits/mango/mango-tree-diseases.htm

You may have an aphid problem that is attracting the ants. Here is help:

https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-aphids-and-ants.htm

It is far too young to pinch without knowledge on how to care for a low branching tree! Pinching now will make branching start too low to the soil level, which will invite pests, and disease.

Pinching, or cutting off the very tip of a plant can help structure, but you will want to do this at least 3 or 4 feet off of the ground to make sure that it has enough airflow close to the root zone.

Topping should occur after the first or second year when grown to its full potential. If it doesn't reach 3 or 4 feet by next year, then you may want to look into lighting the tree with horticultural lighting, as it will mean that the tree does not have enough light to grow to its potential.

Pinching this young will be reserved for Bonsai specimens, and will require quite a bit of knowledge on caring for this type of container grown tree, so it isn't recommended if you only have one tree to experiment on.

Here is an article that will help you to care for Mango in container: https://www.gardeningknowhow.com/edible/fruits/mango/mango-trees-in-pots.htm

Although I am not familiar with the climate of your country, a quick search reveals a mild climate with lows that do not fall below what a Mango can handle.

You should be able to proceed with planting.

Here is an article that will help you to care for these trees outdoors: https://www.gardeningknowhow.com/edible/fruits/mango/growing-mango-trees.htm