Given the current "crisis", I wonder if older off-the-shelf NAS from QNAP, Synology, Asus, UGreen etc. could be re-flashed using FOSS.
The ARM models tend to be very very very slow and small and have architectures that are obsoleted in many Linux distributions combined with hardware that's weird enough that NetBSD doesn't support it (e.g. LAN chip or SATA, which you might need in a NAS). And yes, looking at NetBSD pages and build configs is one avenue for finding well supported devices. Although the documentation isn't great or complete. Some models may also have weird boot sequences or rescue modes that are extra hassle.
I keep wondering if a neat little buildroot config would work with some of these devices. Other odd options would be https://armbian.com/boards and https://openwrt.org/supported_devices .