You don't have to do it yourself. Why not just buy one of these:
https://www.ixsystems.com/freenas-mini/
I have won and its pretty awesome, running for years, have replaced the disks multible times by now.
Its very upgradable as well, you can put in more ram, caches and so on.
Open Hardware is pretty much a no-go at the moment, but iXSystems does support a lot of Open Source.
I would recommend this setup above any of these Synology things, when I compare my problems with my friends who have Synology, I usually come out pretty happy with my choices.
If RISC-V is fast enough to power a NAS and open source SATA controllers exist, I would absolutely prefer a setup involving that.
If I'm not going to get that... I'd probably prefer a setup from Synology. They'll do a good enough job, probably better than I could, even if the price is a premium.