If I understand, i think you are running a web server & kodi on the RPi. In addition you have a PC running as an OMV server. The OMV server has your data/storage. Is this correct?
You should move the web server from RPi to OMV - there is a web server plugin called nginx in network section of plugins. You will have better web server performance on OMV and then you can run rasplex on your pi for the TV.
You can use a RPi v3 as a plexserver but i would not do this! It might work ok but performance will be bad i think. Maybe better to wait for RPi v4 then it will be powerful enough! Maybe USB 3, faster network etc.
I recommend that you install plexserver plugin on OMV. Get it working nicely. use this to stream on your android phone. when/if you are happy with this, then try rasplex on your RPi
With plexserver running on OMV you do not need VPN to connect. It is designed to work over the internet with ddns.