Again: bad idea unless you fully understand which file/folder metadata are stored in which way by OMV (Samba and/or Netatalk) and macOS on 'foreign filesystems' and how encoding conventions look like. Unless you use only plain ASCII for file and foldernames and none of your files has metadata attached (impossible) this won't work well.
Use a disk formatted with a POSIX compliant filesystem on your OMV box and access it only through the network (with either Samba or Netatalk -- see above for the reasons).
BTW: it seems you're using a Rasperry Pi since you fear low NAS performance? If that's the case... there's no reason to run OMV on such crappy hardware. You get faster and even less expensive ARM boards: Which energy efficient ARM platform to choose?
I might be wrong but I'm confident ext4 is POSIX compliant. This is how I would format the hdd to create a shared folder.
Interestingly enough, I just connected a 1TB HFS+ FS hdd up and it works perfectly fine... It showed up in the device list and I can just simply share the folder as I wished. I don't now think I will need to format to ext4 as it would seem that I have a hdd that can be local and shared, not that I need this at all...
No idea why this hdd works when the other didn't. I got given the Raspberry Pi, very kindly, by a friend so there was no cost; hence, I am using that.
Thank you for your assistance and advice!