I've bought a Synology DS415+ back in December 2014. So it just turned 9 and it's still kicking. (Even with the C2000 fix.)
Although Synology stopped delivering updates, I'll keep it as long as it does what I need it to. However, my next device will be a TerraMaster where I'll install OMV on. Can't get a NAS with custom OS in a smaller form factor.
I'm looking at the TerraMaster F4-423 which is basically an Intel NUC soldered to a SATA controller. It has 4x 3.5" SATA bays, an internal USB slot for the OS, 2x m.2 slots, HDMI output, 2x 2.5G LAN, etc. Comes with 4GB RAM, supports up to 32GB. I think it's the smallest NAS with custom OS you can get.