Hi there, reporting back after a while. So I did solve it, but not with any of the recommended solutions. In the end I developed my own (kinda) ETL. Just a Debian server with a MySQL database, some scripts and an FTP server. The data is exchanged via CSV files. Super easy to use with my other systems.
I then asked an Odoo partner to arrange for the im- and export in Odoo and they just use the native im- and export feature of Odoo via CSV. So I can test any format of data by simply importing a CSV file by hand or arranging my own export template.
The trick is not my little server shoving data around but using the native functionality of Odoo. I therefore could have used any other ready-made system as long as someone arranges the automatic im- and export in Odoo.
The disadvantage of not having a fully integrated ETL is that the Odoo partner has to arrange for the automated im- and export over the FTP server. But I can live with that, as my demands do not change very often.
Hopefully that helps someone and maybe we get a fully fletched ETL making use of the Odoo API one day 😀