Fujitsu has abandoned customers of its NAS servers a long time ago. The Fujitsu Celvin NAS lines are very similar to QNAP. I would like to liberate the hardware and get a handle on it with a somewhat modern platform. It’s common to install software of the QNAP ts-219 on Celvin q700 devices because QNAP continued supporting their firmware for 6 years longer than Fujitsu. But that’s still proprietary. In the end free software is ideal.
There are some articles on putting Debian 10 on these old NAS devices – and no later than that because the Debian 11 kernel exceeds the 16mb capacity of the (apparently hardwired) SD card.
I noticed Devuan was named as being good for low memory hardware (512mb in my case). Devuan also appears to support armel (unlike antiX). So I would expect Devuan to be better than the normal stock Debian. Is that sensible? Why are there no articles of people installing Devuan on their NAS devices?
Could you restore the original NAS software if installing Devuan failed or did not work as well? If you can, I don’t see a reason not to try it. Your post is 30 days old at this point - did you try it? did it work?