Exactly the same iMac 15,1. Now running Sonoma 14.1.1 with OCLP 1.2.1 release version. But... you must have a 2 USB port Apple wired keyboard (acts as a hub) and a wired mouse, or a USB hub to connect a wired mouse and keyboard to in order to get through the Apple install. Without the wired mouse and keyboard, everything is sooooo slooooow, makes it almost impossible to get through the Apple setup screens. After the OCLP root patches, you can go back to using a bluetooth keyboard and/or mouse.
Performance is quite good with Sonoma. The iMac 15,1 will also require an initial install with OCLP for Monterey, but it doesn't need any root patches, and the regular bluetooth mouse and keyboard work even during the install. If you are wondering what to do, you might try Monterey on a separate Volume.
I have had no similar screen flickering, using the "Display P3" setting.