I may have misunderstood. Just for clarity, be sure to wait until after the water softener has cycled. Ours is set to purge at 3 AM. Any water prior to that will still be rusty.
I added an inline canister filter from Lowes or Home Depot between the water softener and the darkroom sink. I have 3 and 1 micron filter choices. This was to get those final little dark spots rather than the whole rush of rusty covered by the water softener. It slows the flow considerably, but is worth the wait at our house out in the country. I turn it off to regain normal speed when I am not in the darkroom. We run drinking water through a Brita in the kitchen.
Looking forward to meeting you in MI.