I poured a slab in my garage last summer, mine is about 5" thick at a
minimum. From the research I did, 1 1/2" is pushing it, and I would be
hesitant to go that thin. Can you maintain some of that pitch, and
give yourself and extra inch in the high spot? You'd probably never
notice such a slight pitch in the floor, it would also give you some
drain pitch (might not be necessary) I pitched mine toward the garage
doors. Also, I'd suggest you get fiber reinforced concrete.