It looks like its creating a new process and going in the background and systemd cant track it anymore, so it thinks that its exited and tries restarting. I took a link Oscar sent, and I saw that there is a systemd service and the Type is set to forking, I think this could solve the problem, they also have an ExecStop line, id set it to ExecStop=fusermount -u %h/googledrive so it will unmount properly whenever you manually stop the service. So try setting Type=forking, and adding the ExecStop line, hopefully this will stop systemd from restarting it when it hasnt actually exited

I dont think systemd will expand the ~, try replacing the ExecStart=/bin/bash ~/.local/bin/ocamlfuseStartup.sh line with ExecStart=/bin/bash %h/.local/bin/ocamlfuseStartup.sh, this will expand to your home directory, if its still giving a not found error, try running which google-drive-ocamlfuse in a terminal and make sure the path is correct

The 203 error you got is because your script isnt a valid executable, it needs to have a shebang at the top, you can change it to something like this and set the executable bit with chmod +x <file>

#!/usr/bin/env bash
google-drive-ocamlfuse ~/googledrive

this tells it to run using bash as the interpreter.

Im not familliar with this google drive software, but im figuring that its exiting with an error code cuz its running as a system service, and $HOME probobly isnt set so ~ doesnt expand and the software gets an invalid path.

But I recommend using a user service for this, it will run when you login, you should be able to copy the service file you already have into ~/.config/systemd/user/ and run systemctl --user daemon-reload and systemctl --user enable startup.service --now, this will enable and start the service in one go.

I also recommend adding the following lines under [Service]

Type=simple
Restart=always
RestartSec=60

idk if the software will exit if it loses network or wifi or anything, but this will have it automatically restart after 60 seconds, should it exit for any reason.

If you need it to run before login, it is possible to do with a system service, but it will need a bit more setup