I'm also getting flickering after upgrading to Big Sur. I've had similar flicking in the past with Chrome / Electron apps but was able to address that by adjusting GPU settings for those but with Big Sur it's come back even with those settings. Happens on my external monitor over usb-c, it seems to happen most when there's more 2d rendering happening on screen - typically a combination of Slack, VS Code and Chrome will start to trigger it. Changing what's on screen seems to address it so maybe some kind of buffer thing.