Do I have to keep studying until I hit or exceed my review threshold?
No, you can stop whenever you like. The review threshold is there to give you an idea of what awaits you the next day, but that doesn’t mean you have to max out every day. Go at a pace that feels right for you. If you notice that you stop consistently below the threshold currently set, you can hit the Configure button and decrease the threshold to match closer to what you’re comfortable with.