Well in that screenshot 6 of your cores are below 90% C7-state (which is the sleep state) meaning you have 6 cores over 10% active which is quite a lot. I don't know how the c-states exactly correlate to boosting behaviour but I can get all my cores except the one being benched to over 90% C7...
As per my edit: background tasks can keep the CPU busy enough to prevent higher boosting. You can check task manager for potential culprits. Also monitor your C0% in TS while the PC is otherwise idle. The lower the better.
As you can see in the Turbo Ratio Limits section your CPU will only boost to 5.1 Ghz if 1 or 2 cores are active. That means any multithreaded workload like the 16 threaded benchmark you tried will only ever boost to 4.7 Ghz. That is completely normal behaviour for Intel CPUs.
Edit: Also a lot...
One thing to note: looking at hwBot, an overclocked 4080 can get over 10000 points, which means most of the preformance increase came from the core oc and not from the memory replacement.
C0% is relative (as the % indicates). So if the cpu runs at reduced clocks (800Mhz) from speed shift, it takes a larger percentage of the remaining clock cycles to do the same background work than when it runs at full 4Ghz+.
If the lines show up in a screenshot, like posted in OP, it can't be the cable because the screenshot is taken directly from the GPU output.
I have also seen a similar post in the steam forums where this screenshot was posted. Since the black line clearly disappears behind other in-game objects...
One important warning for your suggested RX 6000 options:
The CPU OP has in the system only supports PCIe 2.0 and the lower tier 6000 series cards have cut down on PCIe lanes (6400 especially with only 4 lanes) which would impose severe bandwidth limitations. Not sure how relevant that would be...
Since I also have a 1660 Super I tested the new build:
While Resizable Bar says "Disabled" now, GPU hardware support is still saying "Yes". That should not be the case, right?
Realtemp is Intel only and not supported anymore since Sandy Bridge if you check the changelog (latest version 2012). I recommend using HWinfo. Alternatively Ryzen Master also shows temps.
Make sure the desktop only has shortcuts or links on it. Had to troubleshoot a laptop once that had gigabytes of pictures in a folder on the desktop. Prefetching all that took forever and sometimes even ran out of resources while booting.