Workout wise you need something a lot more even, weights sessions that work the whole body, there is a routine on a sticky from goldfish in the weight training section that is balanced.
The variation of doing intervals and weights is good. I would advise adding in some longer duration cardio to your weekly program, this could be on machines or take the form of circuit training style workouts.

Burning fat is not a targeted process, you will lose it over your entire body. The belly fat will go as part of this but if you lose 10 pounds of fat only a couple at most will be from the belly. There is a lot of money to be made selling solutions that promise otherwise, but with the exception of surgery there is no process that removes only belly fat.

I am going to assume by your declaration that your diet is fine and controlled this means in good balance with sensible mix of complex carbs, fats and proteins for energy. If this is the case well done but it will raise a question. If you have a well balanced, controlled diet, why have you destroyed the balance with supplements? The term supplement means to compensate for something missing, a good diet never needs supplements, there are no exceptions.
Adding additional protein to a well balanced diet will result in fat gains unless you increase activity to burn the extra energy this consists of. Most people don’t think of protein as calories because we are told muscle is made of it. Protein is 4 calories a gram, the same as carbohydrate, and just under half as calorie dense as fat. So when you add protein your body doesn’t need you are putting in calories you don’t need, it gets worse too.
Fat can be but often isn’t burned before being put to stores. Carbohydrates can be burned or stored with glycogen before the body will try sending the excess to fat store, it would prefer to use this as primary fuel, the best type is complex starch not sugar. Protein that is absorbed into the body cannot be used as energy immediately it has to be converted to something else first. You will catabolise muscle when you train, this means using muscle tissue for fuel, the more you train the less this happens but it happens to everyone, this is one way protein is used as energy. the other comes down to the fact the body doesn’t like waste, if you put in more protein than you need any excess that can be converted to lipoprotein (fat) will be and this goes straight to store.
It doesn’t matter what the label says about cutting, building etc. if your diet is balanced with all you need and you add protein you might as well eat just under half the volume of fat, it will be cheaper and do the same.

Advise on supplements. If your diet is balanced and controlled put them in the cupboard and leave them there until a day when you have eaten nothing but carbs and fat all day.

Gaining weight when you first start at the gym. Totally personal, some do others don’t, the body has to adapt to sudden change, which can mean dumping or holding more water. People often change diet which can influence this or other factors to change weight. Problem is the scales are the least effective way to monitor progress, so I would say use Goldfish’s standard system of weekly photos. I’ve been doing that this year because I am doing aesthetic training for the first time after years of functional, so far I have taken two, which would be great if it was mid January. I get the training done most of the time but my monitoring sucks.