Not really sure that you would need SSD for that, unless your trying to search the directory directly for items.
Normally you would create an index or database of the related files which is stored on flash or ram and that database then just opens the file. If you already know the destination there will not be any noticeable delay between it opening from a HDD or SSD.
In your case what matters is the performance of the Roon database of which your best bet is to invest in loading it into RAM for the highest possible performance using a program like Primo Cache or Primo Ramdisk.
So the real question is what the database size for the Roon database and having enough ram to host the database on a ram disk. This will result in the highest possible performance.
You can eliminate all of those SSDs except for your boot disk.
Your also are just operating a glorified file server so unless Roon has some high CPU requirement you could be just fine with a Ryzan 3 and an OC.