@ljpp we are looking at this, but I just wanted to double check something in the meantime. If you put a link to https://example.com/latest?truendo_open=cookie_settings in a post on your forum and click it, does Truendo detect it correctly and open the preferences?
I suspect the answer is no, because navigations within Discourse are not full page reloads, and I doubt truendo listens to ‘soft navigations’ within a PWA. But I’d love to be wrong on this! 
If it doesn’t work, then I think you’d need to use some kind of Javascript API from Truendo to pop the preferences open, instead of relying on the magic link.