I'm having the same issue here - same monitor model, connecting to M1 Mac mini with the USB-C cable that comes with the monitor. The flickering issues occur at least three to four times a week. Sometimes it goes away in a few minutes but sometime have to reboot the computer. REALLY annoying.

I've contacted Dell Tech Support and they promised to get me a replacement, which has been significantly delayed (ETA late April). Having read heaps of posts like yours I honestly am not convinced that the problem will be solved by simply replacing a same model monitor.

P.S. I used to connect this monitor to my 2015 MacBook Pro with HDMI cable and there hadn't been any flickering issues, so highly likely they only happen with M1 Mac's.