Did you look at the output of journalctl -u docker? The docker package failing to start isn't even the fault of OMV. OMV 5 is every bit as good as OMV 4. Volker has mentioned many times that the 4 to 5 upgrade is not supported. If you are willing to move to FreeNAS, why not consider a fresh install of OMV 5 first??
i installed a fresh version in a virtual box and suffered exactly the same problems.
losing plex was a disappointment. i preferred the plugin over docker. that's why i installed and removed docker in OMV4.
when i did a virtual setup of freenas, the plugins went first time without issue.
don't get me wrong, you guys have done a fabulous job over the years. i've gone from OMV2 through to the OMV 5 and aside from one disaster with the an initial raid failure (which moved me to rsync backups) she has worked liked a charm.