Wrt amount of DRAM I already tried to explain. If your NAS does not do also other 'foreign jobs' (like being used as Docker container host) 512GB or even 256GB are just fine for home usage and the only situation where more DRAM results in higher performance is when the following criteria are met
storage slower than network (eg. USB2 disk and Gigabit Ethernet)
amount of data a client writes to the NAS less than amount of physical memory
Only in this situation more DRAM allows for faster copies from client to server (it's only in this direction!). All the DRAM in your NAS box will end up being used for filesystem buffer/caches anyway but in home situations this is pretty much irrelevant unlike professional servers with hundreds of users or applications that need as much RAM as possible.
Thank you. I understood it that time! I appreciate all the great insights here.