Sounds like you don’t actually have all cores overclocked to 5.3GHz and that your CPU is running at stock frequencies that are lower for sustained multicore use.
eg per info found here https://www.anandtech.com/show/15785/the-intel-comet-lake-review-skylake-we-go-again shows that the I9 10900K in stock configuration will run at 4.9GHz sustained multicore (which FS2020 will operate at 4.9GHz with sufficient thermal headroom, which it appears is what you are achieving.
Check your BIOS that you are actually overclocking.