R5 3600, 2 CCX - 0 and 1, the best core unfortunately is on CCX1, I've never seen it go 4200 MHz, moreover - the fastest core on CCX0 has higher turbo frequency than the core marked as the fastest of the whole CPU.
Windows 1909, 1.0.0.3ABBA - it loads the first CCX (CCX0) cores first, only after all 3 cores (w/o SMT) are loaded - it loads the fastest CPU core which is on CCX1. And it starts using SMT only after there are more threads than there are unloaded cores left, for me it's when I run Cinebench R20 with more than 6 threads.
If using CCX1 first drops overall performance - then it makes sense to use CCX0 first. And then this custom power plan only adds a bit of frequency, the rest stays the same visually, I've not benchmarked it, only checked how it loads cores/threads in Task Manager.
I wouldn't sweat too much over whats marked as the fastest core on the CCD. I'm in the situation too where the supposedly fastest core is on CCX1. For me, both the fastest core of CCX0 and CCX1 seem to boost to 4650mhz under light loads over time according to hwinfo polling at 500hz. I'd say the chance of your fastest cores being so different that it will actually matter is slim to none.