Everything below is my own personal opinion. It is not meant to insult or demean, so please accept it in the manner intended.
I'm of the opinion that Raspberry Pi (up and including the 3B+) is inadequate to serve as a NAS (in the truest sense of the word). I say this mostly because of the throughput restrictions of the USB ports. If you're hell-bent on using a SBC, I'd get something more substantial with USB 3.0 ports. There are a number of candidates that run from $45-120 that would make a more viable file server box.
On the other hand, I would seriously consider building a minimal PC (dual-core AMD AM3/AM3+ CPU, 4gb of RAM, and a motherboard with 4-6 SATA ports), and then put Linux Mint on it. If you want to go really light-weight, you could use Lubuntu instead of Mint, but you'll have to work a little harder at making it a viable file server. It's lacking some of the software needed to "make it go", but that's easily remedied by installing the missing apps.
Why do I say Mint (or any other plain Linux distro) instead of OMV?
0) Because OMV is NOT tolerant of what it sees as dicey hardware. I initially tried OMV on a reasonably new SSD, and it told me there were bad sectors on the drive. When I ran a check on the drive, it came back with no bad sectors. OMV would only run until it tried to write one of its log files, and then it would simply not recover from what it thought was a bad sector, presenting an endless chain of the same error, and because it was doing this, the web admin page presented a 500 error. NO OTHER LINUX DISTRIBUTION had a problem with the drive.
1) OMV had issues with booting. It kept telling me that ALL of the drives in the system "failed the soft reset", but would boot and everything appeared to be fine. NO OTHER LINUX DISTRIBUTION complains like that.
2) Configuring OMV is needlessly complex. I tried for a week to properly configure the shares. I seemed to get halfway there, but I never succeeded.
3) OMV doesn't support NTFS. I'm in the process of migrating 11 machines on my home network from Windows 7 to Linux. As you might guess, my old file server had NTFS drives in it. I had to shuttle files around and reformat the media drives to EXT4 so that OMV work. This process took FOUR DAYS.
Finally, I just got fed up, and installed Mint on the box. Five minutes later, I could see all of the shares on every machine on the network. Having been a Windows user since Windows 2.0, I am very accustomed to stuff that "just works", because I have stuff to actually accomplish.
For what it's worth, my file server is fairly huge . It's in a case that can hold up to 18 drives, has a quad-core CPU and 16GB of DDR3 RAM, and right now, has one 500GB drive, four 2TB drives, and a 3TB drive, and boots from a 32GB SSD. It serves media files to a NUC sitting in my living room that runs KODI.
Advice: If you're on a small home network with no user access issues to be concerned with, creating a simple file server is MUCH easier and more appropriate than setting up a "NAS".
Having said all that...
I have three Raspberry Pi 3B+ boards. Two of them run Pi-hole, and another will be running Pandora FMS when the thumb drive I ordered arrives. I love my Pi's, but I would never consider using them as a file server, or as a NAS. They just don't preform well enough. However, you shouldn't have a problem making your Pi a file server, but don't expect anything resembling a high-performance solution.