On boards where storage is not the bottleneck (Rock64 with USB3 for example or some other ARM platforms) you don't need to worry about that. 1GB will be as fast as 4GB.
With ZFS a lot of RAM is a good idea (but you can run it also on system with less RAM but need to adjust some tunables then, mostly related again to filesystem buffers/caches). But ZFS on ARM is currently 'experts only' at least with OMV. The ZFS plugin is not available for ARM and building the necessary modules in an automated way is also challenging. So better forget about that.
Wrt plex server... no idea, only used that once to demonstrate how easy armhf software can be installed on an arm64 distro.
The post was edited 1 time, last by tkaiser ().