I'm looking for an RSS reader that would be able to have its data synchronised across devices using Syncthing (it just should store its data locally in a certain folder that looks the same on all devices).
What are you hoping to sync? If it is just your subscription list and every device fetches by itself it shouldn't be too hard. However more complex things like read/unread would be hard to sync using a basic tool like Syncthing. The problem is that tools like Syncthing don't really have any conflict resolution capabilities so apps would need to be written specifically for that form of syncing.
Most open source readers use a shared backend server that manages checking feeds and syncing read events rather than this filesystem level syncing.
I do want to have my read statuses synced. I don't plan on reading feeds on all my devices at the same time and thus I believe Syncthing would be sufficient.
The bad thing I see with server-dependent syncing is that you have to either host it yourself (not an option for me at the moment) or trust someone else with your data (not a problem but I prefer to avoid that when possible)
don’t plan on reading feeds on all my devices at the same time and thus I believe Syncthing would be sufficient.
The problem with this is that you accidentally read on two devices before they sync, Syncthing renamed both files and now you need to manually fix it toget your read status back.
I get that it can work well enough, but I wouldn't want to try and build something based on this.
I get that you want to avoid a server. But I'm not aware of anything in this space. It would be an interesting to create a reader like this that has a custom P2P synchronization protocol.