FreeNAS is heavily targeted at the Enterprise sector, while OMV is more geared towards SME and private house holds. FreeNAS uses ZFS as its only file system for everything, while OMV does not. For OMV ZFS is optional, so you can just create ext4/XFS file systems for your shares if you want to. So if you don't want to use ZFS, then this is entirely possible with OMV, but not FreeNAS.
I know that the RPi alone would be slow. This is why I mentioned the Computing Module 4 - or CM4. With the CM4 you have a native PCIe slot, which you can use to plugin a real SATA controller. With that kind of setup I/O speed increases heavily.
Yes, you would need to build probably your own enclosure, until the market gets traction and you can just buy them off the shelf.
But the nice thing about OMV is: if you don't want to use the Raspberry Pi, then you don't have to, because it also runs on X86 without problems, so any Intel and AMD CPU you can imagine.
Anyway, you should never build your NAS around USB only. This is quite dangerous.