Hello! I hope this helps, if you are still not clear on this, please let me know and I will rewrite this to make more sense.
In app.yml set:
env:
DISCOURSE_S3_CDN_URL: [BUNNY CDN]
Then after the app was rebuilt, we ran:
sudo ./launcher enter app
rake s3:upload_assets
exit
[!] I can confirm, Hetzner Object Storage does work with S3.