How to make a strong mug of instant Coffee?
What is too bitter is likely to be primarily opinion-based. That said, when I make instant coffee, I usually use one teaspoon per 200-250 ml of water - not milk. The result is a nice, even cup of coffee, which is not what I would characterize as strong. If I want strong coffee, I will use as much as four teaspoons to the same amount of water.

The milk will have a mellowing effect on the end result. In combination with the sugar, it has been my experience that the milk masks the bitter taste of coffee that many people do not enjoy.

I would recommend playing around with how much coffee you use. Once you get the strength you want, you can add or remove sugar to reduce the bitterness.