Dear Jarem and Snoopy
Thank you for your reply!
In the meantime I was in contact with Threema support.
In summary: notification settings have to be set in the Threema app itself. Then notification channels could be created based on a combination of settings instead of per-chat basis so that no private data is passsed on through these notification channels. Currently there is no DND override option, but they will consider adding this in future.
Snoopy: You are absolutely right that the purpose of DND is no acoustic notifications. However I like the possibility in Android to set an DND override for specific contacts and/or apps. I don't want to be disturbed by customers calling or texting me in the evening or on the weekends. But I don't mind some specific people calling me, and I don't mind the people I use Threema with, calling or texting me either...
Text notifications indeed only 1x every 30 seconds by design. My mistake... and absolutely fine as it is.
Best regards
Christophe