Some WebDAV server, can be Nextcloud but actually something more lightweight is better.
Also a XMPP server is very nice to have. Even if you don’t have many contacts on it (yet), it works very well has a notification service and can even be extended to act as a Unified Push distributor.
An the Syncserver still runs on Python2 with multiple known vulnerabilities.
There is a new Syncserver written in Rust, but it seems in continous half finished state or so.