with a NAS i tend to go with a commercial product, and only for that purpose. it stores the data, maybe serves it up as file server. thats the NAS one job.
my processing happens on another box, like your pi. i want the nas focused on its single purpose.
so my suggestion would be to pickup a netgear/synology whatever, but only use it as a nas.
if you want to expand that pi, just use a real machine and upgrade yourself to maybe a nice docker setup.