Hi @karol,
In the default platform, there is a REST API at /api/completion/v1/subsection-completion/{username}/{course_key}/{subsection_id} (ref) which can give you some completion status for each subsection in a course, but you must make a separate request for each subsection as far as I know.
That said, if you install the optional completion aggregator plugin, then it provides an API that gives you a breakdown of the user’s progress in a course, or across all their courses. However, configuring and running completion aggregator in production is a bit of work (you need to schedule a couple cron jobs to run to update the data), and can be resource intensive.
Also, since the plugin is open source (developed by OpenCraft), you’re welcome to make pull requests as needed, to enhance its functionality, without having to take on maintenance of the whole thing 