Dude, you went 500-1000kcal in surplus for about 2 months to gain those fats now you expect to lose all that 10kg in 3 weeks by going slightly under maintenance (I suppose that means 100-200kcal)? That should have made sense to you.

Doing whatever program ALONE is not gonna help you to lose the belly fat, and more bad news, belly fat is usually the last to go. If that belly fat is your only concern, then calories in < calories out is what you need to watch out for. If you wanna focus on getting strong, then you can continue to bulk.
I don't think you should be focused on a cut right now as you are only less than 2 months into SL5x5, maybe just eat slightly over maintenance and make sure you get enough protein.