I've got a wireless MM, and yeah, the scroll ball gets gummed up after a while. I found a website that detailed the disassembly and cleaning of said component. It's finnicky work, but I can do it fairly quickly, now.

OTOH, as far as I understand it, any Bluetooth mouse would work without the need for a dongle, assuming your iMac has BT built in.
If you're not outraged, you're not paying attention...