It’s not always possible but it’s generally good practice to configure your applications to use external storage rather than file systems - MySQL/PostgreSQL for indexable data, and S3-clones like MinIO for blob storage.
One major reason for this is that these systems generally have data replication and fall over redundancy built-in. So you can have two or more physical servers, have an instance of each type of server on each, and have these stay synchronized. If one server goes down, the disks crash, or you need to upgrade, you can easily rebuild a set of redundant servers without downtime, and all you need to do is save the configurations (and take notes!)
Like I said, not always possible, but in general the more an application needs to store “user data”, the more likely it is it has the ability to use one of the above as a backend storage system. That will reduce, significantly, the amount of application servers that need to be backed up, and may reduce your need to consider using NFS etc to separate the data.
I’m not directly familiar with either, but syncthing seems to be about backing up, so I’m not entirely surprised it’s file oriented, and jellyfin doesn’t look like it’s about user maintained content so much as being a server of content. So I’m not entirely surprised neither would support S3/Minio.
Yeah it took me a while to realize what S3 is intended to be too. But you’ll find “Blob storage” now a major part of most cloud providers, whether they support the S3 protocol (which is Amazon’s) or their own, and it’s to be used precisely the way we’re talking about: user data. Things clicked for me when I was reading the DoveCot manuals and found S3 was supported as a first class back-end storage system like maildir.
I’m old though, I’m used to this kind of thing being done (badly) by NFS et al…