Approach 1: Using a cronjob to manually copy the certificate

1. Make sure syncthing has the https-key.pem and https-cert.pem files present in it’s home directory; my commands assume the directory is /home/syncthing/.config/syncthing [that’s my setup). Make sure the permissions are correct, meaning the files are owned by the user running syncthing. The easiest way to achieve this is by deleting the current files while syncthing is stopped.
2. Upon the next start, syncthing will re-generate the https-key.pem and https-cert.pem files with the correct permissions (files are owned by user running syncthing). Now, you only need to overwrite the files - overwriting existing files does not change their permissions.
3. Open a shell/terminal on the machine, preferably as root or any other user that definetly has access to all certificates inside of /etc/letsencrypt. You can get root either by typing in su or by prefixing the following command with sudo.
4. Type crontab -e to edit the crontab of the current user
   The file will be opened with some text editor, like nano. In the file, below the comments you can add the following lines:
   @daily cp /etc/letsencrypt/live/[domain]/privkey.pem /home/syncthing/.config/syncthing/https-key.pem
   @daily cp /etc/letsencrypt/live/[domain]/fullchain.pem /home/syncthing/.config/syncthing/https-cert.pem
5. This would copy the certificates from the let’s encrypt directory daily to the syncthing directory, overwriting existing files but without modifying file permissions. The solution is simple, but definetly not the best. The @daily should be supported by pretty much every standard cron