Yes you're right Hélène. That's what I already do with central2pg. That way I can ask PostgreSQL to dynamically create the tables and add new attributes when forms evolve.
Using $expand make it impossible even if I flatten the json.
Our main form contains two nested repeat groups.
Plpython and pyODK might make the process lighter than using "copy from curl".
At the moment I can do with Python what I already do with central2pg.
But I need to find a way to only get recent data from subtables. As we discuss with @Matthew_White the easiest way for me would be to be able to filter subtables data on creation/submission date. Getting all data works really well but I am not satisfied to get each hour the 20000 datas collected since we published the form in March when only 100 were collected today.
I'll spend some more time on in a few days.