I'm having the exact same issues. I also have the 27" 2017 iMac (iMac18,3) with 4.2 GHz Quad-Core Intel Core i7, though these problems seem to extend to other models as well. Like many others here, I'm also running multiple Adobe apps including Photoshop, After Effects, and Illustrator, and the issue definitely seems worse while working in those. I've also observed that it gets worse with mouse movements and/or keyboard input (for example, I frequently use Cmd+Tab to switch apps, and I often see intense flicker & drop-shadow-esque artifacts when switching in this way). It seems likely that Adobe is at least partially accountable, but I never saw anything like this before updating to Big Sur.