Photon shot noise makes a bigger difference than C-RAW vs. RAW ... if you compare to RAW files with another on 400% and ISO 52000 they look as different as another C-RAW. Also C-RAW doesn't eat stars.@YuengLinger, It appears that according to dpreview and other bogs, C-RAW indeed produce a smaller file size without difference on image quality BUT in critical conditions like low light, standard raw can retain better details and if you have to retain some details on a very dard scene, standard raw perform better. As I use the R5 also to make night photography (ie. star trails, milky way, etc.), a smaller raw file size was indeed better as I make panoramic night sky images with exposure bracketing and stiching together many very large files can truly kill my computer processor and take ages to build a full panoramic image...
Sure for astro you want lossless, but personally I still use C-RAW and enjoy 45% reduction in size taking my 1000 subs.
Picking the right ISO that is ideal to avoid dark current noise (1250-1600 for R5) is tons more important.
Also your wish makes no sense. Sure the R6 has more photos per pixel, but more pixels more than make up for it. Just watch Tony's video on noise comparison between the two. R6 is strictly worse for low light if you would downsample 45mp to 20mp. If your request is to automatically downsample 45mp to 20mp and call it M-RAW or whatever, well it's not "raw" anymore per se, best you can do is to retain dynamic range.