The need here is to be able to sync a few "mounts" on multiple devices, desktop and, if possible, mobile (Android + SailfishOS).
There are a few existing tools, such as OwnCloud and NextCloud, which are both absolutely overkill and pretty painful to set up/maintain.
The tool should provide this "mounts" service, with an optional simple web interface, and nothing more.
Ideally, the tool should be a runnable archive/binary, and not rely on interpreters such as PHP, Node, or Python.
Edit: Just to be clear, some additional things that make other questions unrelated:
I'm not searching for a tool that "can" do that, and more.
I'm specifically searching for a tool that can only do synchronization, I'm not even interested in a web UI to manage the files online, and want something as simple as possible to use.