I’m another Aimpoint user, and I believe its a great way to go for many players. Yes, you really should get the level to calibrate yourself. If possible, you should take an in-person class. As @jon says, some people struggle to feel the slope, a good Aimpoint instructor can help you figure out a method that works for you. My personal method is to stop at a spot, close my eyes, and rock left and right just slightly. I’ll feel like its easy to “fall” one way or another, and I’ll have that read in a second or two.
Aimpoint is really good because you’re only evaluating the piece of the green between you and the hole. The slopes beyond the hole, the edges of the green, the horizon in the background, you see them all, and its really hard to eliminate their influence in how you read a putt visually. Aimpoint eliminates all that visual clutter. Plumb-bobbing can provide a reference for a vertical line, but you’re still seeing all of that background.