You can show the current folder icon instead of the Opus logo, for individual windows. Won't help with a grouped taskbar, of course. There used to be a way to change which icon Windows used for pinned apps on the taskbar but I'm not sure if it still works.
If you're willing to ditch all the functionality in Opus because of something as minor as the icon then, well, there's no point arguing about that. That's ridiculous.
Even though I would rather only help if it was a functional reason opposed to the one given, assuming you meant just for on the Taskbar, you can follow the steps under "To set it up" here, with any ico file (there are some other icons included by default too):
I just want to causally mention that right now I have four programs open in my taskbar and three of their icons don't look much more than circles (DOPUS, Firefox, and digiKam). With my 67 year old eyes I wish DOPUS stood out a little more. I'm wondering if you could make the rays around the sun a little more prominent.