How have you installed Syncthing in the first place? Normally, the recommended way is to either use the binary version from the official website, or alternatively follow the instructions listed at https://apt.syncthing.net.
I’m mentioning this, because a) it sounds like something is wrong with your current installation, and b) the versions of Syncthing served by Debian/Ubuntu are heavily outdated and may not play nicely with the newer ones (e.g. like those used in Android, which you’ve also mentioned above). Many bugs have also been fixed since then.
Sure, just make a copy somewhere before uninstalling the current version. Then, once you’ve installed the official release, just copy and paste the files into the newly created Syncthing home folder. Do this while Syncthing is not running.
For the record, you only need the pem files and config.xml. Theoretically, it may be possible to re-use the database (i.e. the index-v0.14.0.db folder), but I would be cautious, especially if your current version is very old.
Not quite sure what this is. If it has something to do with the “Snap” packaging, that might explain the different configurations. Apps installed via snap usually have a separate home directory from what “regular” apps use to store their configuration.
I copied the whole folder. Uninstalled it and reinstalled it and took the chance to copy back the config folder / syncthing folder. Started the service and all my config and index etc etc was there Up and running !