Nice, I also got an Ryzen 7 system running Plex media Server.
A Ryzen 1700 can handle around 7x 1080 bluray 10.000kbps transcodings. I've had up to 12 stream running simultaneously mix of 720p and 1080 without any problems.
Your current build can only handle 3 harddisks since one of the four SATA ports are going to your SSD. And I guess that x16 slot are going to the GPU, which means you only have x1 slots back. You kinda need PCI-E x2-x4 for a decent SATA card.
I would also recommend the Ryzen 1700 over 1700x and 1800x since the both got the same overclocking potential. The Ryzen 1700 even comes with a decent cooler boxed.
The case seems to be a bad purchase for NAS, it only has room for two 3.5" HDDs.
Some changes:
https://ca.pcpartpicker.com/list/3hjMZRMost important the motherboard has 6 SATA ports and got PCI-E lanes for a SATA card.
Didn't pick a case. Not sure if looks is important for you. The most complete NAS case for me is the Node 804, for some reason the price on it in Canada is insane.
You should at least aim for a case which can handle a bunch of HDD's. One made for mATX board would also be a good idea just for the size of the case.