syncthing.sh

# Via: https://sts10.github.io/2018/11/27/syncthing-and-tmux.html
# Changes to `ss` and `se`:
#   - Start session with command on `new-session`: avoids creating a shell
#     session just for `syncthing`; also use only `kill-session` instead of
#     sending keys.
#   - Test if the sessions exist (or don't exist) before creating/killing them.

# in ~/.bash_profile or ~/.bashrc
function ss {
  if tmux has-session -t synct 2>/dev/null; then
    echo "Syncthing session already started." >&2
    return 1
  fi
  
  echo "Starting up Syncthing at http://127.0.0.1:8384/"
  tmux new-session -d -s synct "syncthing -no-browser"
}

function se {
  if ! tmux has-session -t synct 2>/dev/null; then
    echo "No Syncthing session to end." >&2
    return 1
  fi
  
  echo "Stopping Syncthing and killing the tmux session"
  tmux send-keys -t synct C-c
}

Shell script skills only improve with experience, so keep at it :)

Regarding syncthing remaining executing after the kill-session, that's very interesting. I wonder if the signal tmux kill-session is sending (I was unable to find documentation for that quickly) is being ignored by syncthing. Ctrl-C sends a SIGINT, so I'm assuming kill-session sends another signal (maybe SIGTERM?).
If that is the case, then the se function you wrote probably doesn't need the kill-session command: by sending the Ctrl-C the syncthing process exits, and the session should end right away.

Regarding the error message: you can redirect that output (which tmux writes to stderr, file descriptor 2) to /dev/null with the shell redirection 2>/dev/null. I made this change, along with your se fix, to the gist!