I had the same issue last days. Your problem is related to the "Type" field in "[Service]" part. Your are using Type=simple which execute at the beginning without wait for the other resource necessary to use your script. By changing to Type=idle should solve the problem.
idle means that your script will execute after all the service are initiated.
A better approach is by looking the services that your script needs, and also writing the field "After=service_required_by_my_script.service service2_required_by_my_script.service ...and more" in the [Service] part.
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