Bash-script as linux-service won't run, but executed from terminal works perfectly

The problem is that, when the script runs as a service, it does not run as "you": it does not have your environment. Specifically, it does not have your PATH variable.

Either add /home/<usernamehere>/.opam/default/bin to the PATH in your script, or simply hardcode the full path for that program.

The most likely reason is that the directory containing google-drive-ocamlfuse is in the PATH of your login shell, but not in the standard PATH used by systemd.

Just add a line like this at the beginning of your script:

PATH=$PATH:/path/to/google-drive-ocamlfuse