We finally made it work.
I don’t remember how I initially created /etc/opensearch/opensearch.keystore but when I run /opt/opensearch/bin/opensearch-keystore create it puts the new keystore file in /opt/opensearch/config
So I removed all of the credentials from the keystore, made sure it was created in the /opt/opensearch subdirectory on every node, then copied that into /etc/opensearch, reloaded settings via the API, then I was actually able to create a repository and take a snapshot.