Answering my own question:

If you've got a QNAP, you can install Debian 10 on some of them <https://www.cyrius.com/debian/>

If you've got a Synology, it has been done on some older devices as well <https://wiki.debian.org/InstallingDebianOn/Synology>

So all is not necessarily lost, and I have one of each so will need to do some experiments!