There's not going to be any noticeable difference in performance between 5600 and 6400.
In the hardware unboxed video I watched (I think it was: Zen 4 v Raptor Lake memory scaling,
link), there was a 3% difference in average fps between 5200 and 6000 and a 3% difference in average fps between 6000 and 7200. This was at 1080p, with a 4090.
In Guru3D's article (13900K memory scaling,
link), there was a 3% difference in average fps between 4800 and 7200. This was at 1080p, with a 3090.
I'd expect the gap to close at 1440p, to nearly disappear at 4K and to be less noticeable, the slower the graphics card. I think you can see that here, in an older TPU
article with a 12900K and a 3080, there's a 0.2% difference between 5200 and 6000 at 4K.
Are both drives heavy usage? I'd usually suggest you only get high-end drive with DRAM, for the boot drive and apps, especially if one drive is only used for games or light storage. The WD SN770 is still pretty fast and has decent endurance, but is less than £140 for 2TB.