I would recommend going x86 as i have a RasPi4 running plex and samba but now i want to expand and build a new PC too. Its proved seemingly impossible to find a simple way to power 4x 3.5" drives and connect to the RasPi without having 4 power leads for each powered usb to sata adapter and a hub to connect all the USBs together. The Rock Pi seems to be more available but then you still need a full PSU just to power the drives. You can get seperate enclosures or docking stations for drives but they're unnecessarily expensive.
If you're just looking for a simple NAS with couple extra dockers in OMV, then sbc will do plenty and the small size is a bonus, but i think x86 gives you soo many more options