An expansion tank is like a mini bladder tank used in a well system - you can think of it like a personal water tower until it empties, your output volume would be based on your new piping. Size it properly, and your volume issue may go away until you try a large load like maybe filling a big tub or laundry. But, an expansion tank holds VERY little water. To get more volume, you could use an actual bladder tank designed for use with a well. The incoming water is pressurized, and that compresses a bladder, so when you use some, the bladder pushes it out until the tank is essentially empty. What you have is a very low volume, the pressure would be the same with a soda straw sized pipe or a fire hose when there's no flow, but as soon as you increase the flow, the friction slows things down, and your dynamic pressure goes down. There are booster pumps, but one of those needs an adequate incoming volume, but at low pressure, and the pump boosts it. Not reliable if the incoming volume is low, as then the pump could try to be sucking air rather than water through the small pipe.
A better overall situation would be to try to get the condo association to replace their old galvanized line before it starts leaking, and the end result would help everyone without problems. In my condo, something like that would require the association (obviously, made up of the owners), to fix the problem. I suggest you read your bylaws, they can vary considerably from one place to another, so that may not apply. Galvanized piping is not a good idea, and with its limited lifespan, anyone with any should be planning to replace it. The timing would depend on your water quality, the volume, and the age. Sounds like yours is way past its lifetime, and as it rusts from the inside out, it gets thinner and thinner, and leaks can't be all that far away. Iron rust is larger than elemental iron, so as it rusts, the flakes expose additional iron, and the rust continues.