Paint thinner only removes oil base paint that has not ever dried. Don't bother.
Single edge razor blades are the best option. Find a blade holder that is comfortable in your hand, and a different one that you hold comfortably in some other hand position. Switching back and forth between them will reduce the cramping you may feel if you haven't done this regularly.
Another trick is to replace the blades often. In college my wife and I had summer jobs cleaning paint off windows of newly built houses. There you only had to deal with a thin line of paint and a few drips and speckles, but a blade would still go dull often on a single window.
Don't hesitate to replace the blade. Dull blades can scratch glass. You may go through a dozen blades doing the back of a fish tank. If the paint is old, maybe a lot more than that. Toss the blade the first time it begins to drag.