Rock64 home NAS build thoughts?

Hi.

I'm a complete OMV noob.

I've been reading and studying numerous OMV forum threads over the last week or so. I'm preparing to build my first OMV NAS for light (2 user) home use and wanted to solicit experienced users thoughts. I'm most concerned with simplicity, low cost, low power, and easy access. Here's my potential setup:

Rock64 1gb SBC
Sandisk Ultra 32gb micro SD card
WD My Book 4tb HDD (usb 3.0)

The clients include 2 Macbook Pros and potentially a Samsung phone, an iPhone, and a Raspberry Pi or two. The Mac's are currently backed up and cloned onto several drives using Time Machine and Carbon Copy Cloner. That would continue.

While AFP would be a bonus for a Time Machine NAS, if stability (disconnects, crashes) were an issue, that could be sacrificed. In fact, I was thinking of using ZFS and access through the Finder using smb. Drag copying to the OMV NAS would be fine. Outside (WAN) access via dyndns or some such service. Potentially, I might want to add Next/Owncloud for Dropbox like features.

Any thoughts about this setup and potential pitfalls?

Thanks to all in advance.

Yes, I think that version of the SanDisk microSD card is A1 rated. I don't completely understand which option is the best for the OMV system disk - microSD, USB SSD, or ?

As for backups, I will continue backing up my data through other methods. If everything goes well after some months, I may add a second Rock64 NAS which will be a copy of the first NAS using Rsync. Ideally, this NAS would be off site.