So, just tried this trick and it worked…
Adding this line to the JVM parameters in the file /etc/default/graylog-server, specifying my path to the certs store.
I don’t really understand why Graylog is NOT using the default java 17 keystore out of the box, but it’s definitely something to dig…
Browsing 4+ years old topics is definitely not my favourite part but it worked this time ! 
Maybe adding this to the install docs might be a good idea ?