I have a modest configuration, OMV4, Athlon II X2 250 3.0, 6GB RAM DDR3 and 4TB for data.
Use FTP, SMB, CUPs, MySQL and NGINX for Nextcloud.
My C2D (an E7400 btw, just realised now I forgot to say this in my original post) should deliver about the same performance level as your Athlon, so it does give me peace of mind that despite the age, my hardware should be enough for the job.
You need good NICs so if you want to stay with onboard stuff ECS already lost since 'Atheros AR8112 10/100 LAN' -- that's Fast Ethernet and therefore too slow.
Again: CPU horsepower and amount of RAM are 100% irrelevant for your use case, you need a good NIC with some offloading instead.
Intel board it is then. Thanks for the heads-up! And I guess I'll give the Celeron a try, since CPU power is irrelevant, the lower TDP becomes the next best thing.
About the ODROID boards, I had given it some thought, but I just upgraded my rendering/encoding machine recently and I'm not willing to invest any money in hardware at the moment. The HC1 and HC2 are really good products though, and I might pick one up at some point in the future.
Both replies did clarify a lot of stuff for me, thanks everyone for the help!