Hi,
I am in the exact same position - only working on mac except for windows.
Exfat is the only viable way for me to make data accessible on OMV and Mac directly. No idea what kind of speed reduction exfat has over ext4 but I guess Wifi speed will be the bottleneck anyways.
I have followed this guide to set up exfat: [Guide]How to mount ExFat USB Drives Manually and use them on OpenMediaVaults WebUI
With this I can read from exfat in the console but still I am not able to create a shared folder in ovm as the exfat drive is not in the list, although it is in the list off files system. No idea what the problem is...
..starting to wonder if OMV is really a good choice for me. I was hoping to setup a raid system in the end using exfat drives but that seems to be not possible at all. I might just create a samba server instead...