The problem with using seaweedfs to a back your DBs is more on the filesystem than the implementations of POSIX features. When you are writing to a file, and the connection to seaweedfs breaks (container restart, wifi, you name it), then you might end up with a half-written file. If you upload pictures, this is unlikely, but DBs are doing several writes per second usually. So it is more likely one of those gets interrupted. In my case, my grafana sqlite DB would get corrupted every other week.
What I recommend is using DBs natively in your node's filesystem, and backing them up to seaweedfs periodically instead. That way your DBs 'work' but you can get them running again, and the backup is replicated in the distributed filesystem.
Good luck on your Nix journey! Happy to help if you have questions.
Of all the tech I use, I think Nix is the most 'avant-garde' in that it is super different from the usual methods (scripting, stateful things), but works very well once past the paradigm shift and the learning curve that entails.