OMV images can be found here: https://github.com/ayufan-rock64/linux-build/releases
I would prefer the armhf variant (32-bit userland) since it has a lower memory footprint and it's easier to run Plex media server with it. Background info: ayufan and I worked close together to integrate all the improvements for ARM boards and the images are fully supported (kernel updates as deb packages so all you need to do is to 'apt upgrade' from time to time to keep all components of the image up to date).
Does it make sense to install PlexMedia Server? RK3328 is not enough for transcoding, and it can not be disabled.