Galvanized pipes tend to corrode from the insides, but guess what? That corrosion IS the pipe, and eventually, it can do one or both of two bad things: decrease the flow significantly (you're seeing that now I think) but the next stage is it just starts to leak since there's nothing left. It is rusting...eventually it will rust out enough to leak. It is kind of foolish to wait once symptoms start to show. Think of the complaints when the walls and floors need to be replaced from the leaks - much more problematic than fixing it now.
I also live in a condo, and while the owners don't like it, sometimes, you just have to bite the bullet. We've been fairly lucky, but have had to do assessments a couple of times over the last 20-years to fix things we didn't have enough in the reserves to cover. Nobody likes it, but it must be done.
Trying to flush the lines may hasten them leaking, and probably loosen up enough crud to plug up everyones showerheads and valves.
If there's a bad check valve on one branch, or someone has a failed shower mixer or faucet valve - some valves can generate a cross-over situation which could prevent the recirculation system from doing its job...the water will take the path of least resistance. One way to check on sink faucets is to feel both supply lines...only the hot one should be hot. If both are, then the valve has failed and the hot water is crossing over, short-circuiting the recirculation.
Good luck, but I do see a repipe in your future. Having hot water is a requirement, and the condo association is required to provide it. It's more common to have individual WH, but you have what you have, a shared resource...now, you have to get everyone reasonable access, or you may end up getting sued.