Bahmni is a distribution of OpenMRS, that covers the broad hospital workflow. You should definitely consider it!
To your original question, the OpenMRS reporting module has the back-end queries you’d want, and these are accessible via the Reporting REST module. Read more about that API here: https://github.com/openmrs/openmrs-module-reportingrest/wiki
You should also look at the summer project described here: [GSoC-2017] : Built-In Reports for Reference Application (You may want to jump to the end and then scroll backwards until you find a demo or summary.)