So I'm glad you like the Belgian fries. What I've always learned to be key is that we fry our cut potatoes twice: once at 140°C, wait for them to cool down, and once at 175-180°C. Then you add salt, that's it.
And the mayo: I don't think many people like it like that when they eat fries at home, but it's easy ¯\_(ツ)_/¯
FWIW, they're brined in a solution containing sugar, it's not like there's sugar sprinkled on them. Very little of it makes it into the final product and in my experience brined fries aren't perceptibly sweet in an obvious way. It assists with browning and a crunchy exterior.
There are some other variations, like blanching the potatoes with vinegar, or freezing the potatoes overnight between frying. The idea is to make them crispy on the outside and soft on the inside.
But, in any other situation if mayo is involved I'm grossed out to the point of puking. Even when I just smell the mayo (from the same brand) someone is putting on their plate of vegetables or whatever.