This may be a bridge to far for some of us, but I've found forms online where you can take a complaint directly to the national authorities from where the flight originated if you're not satisfied with the airline's response.

Here's the general English complaint form...https://ec.europa.eu/transport/sites...nt_form_en.pdf

Here's the addresses for all the national authorities... https://ec.europa.eu/transport/sites...ent_bodies.pdf

And in my case, leaving from France, here's the form in French from the French authorities. you need to download the PDF by clicking the appropriate place on the top of the form
https://www.formulaires.modernisatio...numCerfa=13675