I inadvertently hijacked another thread as my issue was related to a side point that was mentioned - indexing an AFP share. See (post 37 to 64; AFP transfer sometimes very(!) slow) for several methods that have been tried to spotlight-index an AFP share hosted on OMV3.
So far, the only method which works for me is to store my data in a sparse bundle which is formatted in HFS+, which I then mount within OSX. And then enabling indexing on the mounted sparsebundle with: mdutil /Volumes/<share> -i on
Unfortunately, the mdutil command doesn't work natively on an AFP share (for me). It would be ideal to index the AFP share natively.