Low calories. Idea?

If you want to loose weight a high-protein powder would be good. You could combine the Active version with some protein powder. One scoop would give you as many calories as the conventional shake, but you could add more water per scoop and still get a good satiety.

Let’s say your metabolism is 1700 kcal (which is really low for your weight)
Add 300 kcal per day (which is extremely low)
It gives 2000

Add 4 intense workout per week : 500 kcal * 4 / 7 = +285

2000+285 = 2285 kcal for maintenance

How can you gain weight with 2100?

That depends on your height and activity level. There are several calculators to be found on the internet. If you really want to know you should use several ones and take the arithmetic mean of the recommended numbers. If you do not reach your weight goals after some time, you should reevaluate.

According to the calculator that I use you would lose weight at 2100kcal with your activity level:

I have used the same calculator and I lost 10kg in 2020 while keeping a large part of my strength/muscle.