So I spent a while with Dell Support and Apple support, and neither of them helped me solve the problem. I continued to troubleshoot each time the issue came up until one day when I found the internal screen setting called "Dark Stabiliser" This setting instantly stopped the flickering from happening, but the problem with this setting was that it also created weird highlights with my photo, and video editing which is not helpful so I had to turn this off. I continued with troubleshooting and found that the issue is at its worst when there is a large amount of shadows/darks in a site, image or video on the screen. So I then adjusted my contrast and brightness in the internal screen settings and found I could get that flickering to stop. I have only found this today, so I can't confirm it's a long-term fix, but right now, I cannot recreate the issue the way I could previously, which gives me hope. At this point, I'm running two 4k 120f USB to HDMI cables; in my Apple display settings, I have the colour profile "DELL S3221QS", and in the Dell internal settings, Dell presets >Standard & YPbPr. My contrast and brightness are set to 85% (I don't love that I don't get to have control over my settings, but I'm glad I can enjoy the screens)  Hope this helps someone
As soon as I connected my screens (I have two of the same), I found that when I scroll up and down on particular sites, I get vertical lines and a flashing happening on the screen that I'm scrolling on. I played around with the refresh rate, which only made it worse (Changed from 60 to 50), and then I thought of trying to change the color profile. When I changed the color profile to Apple RGB, the problem disappeared. I'm hoping this fixes the issue long term. I am mostly posting for anyone else that may face this issue.
S3221QS, Mac Studio M1, flickers when scrolling