The answer to the question about the leak rate changing is yes. The expansion tank likely has a crack up the side. If so, when it is cold and has no pressure, the crack closes nearly water-tight. When the pressure in the system builds and the side of the tank bulges out, the crack opens up and leaks a lot.
If you want to try to replace just the expansion tank, it is designed to be removed without removing the radiator from the car. In fact, the radiator is a heck of a lot easier to get out after the expansion tank has been removed. However, after this many years, you have to be really careful about how you get the expansion tank loose without breaking any of the brittle plastic parts, and even more careful about putting the new one in. The first thing to remove and the last thing to put back is the water level sensor (1/4 turn to release from bottom of tank) because it is a very fragile part.
On the last expansion tank that I put in, the plastic molding was imperfect where the water level sensor went in. It looked OK but the sensor would not fit until I filed down the plastic a little with a small file.