I would like to know if anyone has used TOTAL calories in Activity to maintain their weight.
Not me, because when I started losing my first thirty pounds back in 2009-2010, the Activity app didn't exist yet. I was using LoseIt! at the time, and haven't yet compared Activity's recommendations with LoseIt!'s. But, yes, when I was tracking calories, I used LoseIt's total calorie count to determine how much to eat. If I didn't work out that day, I ate less; if I worked out, I ate more.
What I learned, though, was from reading between the lines, so to speak -- I could quickly max out my calories with potato chips, candy, and sodas, or I could eat a lot of fresh veggies and other unmolested foods, getting good nutrition and a greater amount of food while staying within my calorie budget.