from the Couchbase documentation I didn’t find a proper role to grant a specific user the Indexes read only permission.
All the permissions include a sort of workaround to manage the indexes.
Is there a role, that I maybe missed, which can grant the indexes visibility in the Couchbase UI, without the permission to manage them?
Did you try Read Only Admin permissions in general section. That allows to REST endpoint
curl -u usr1:password http://localhost:9102/getIndexStatus which UI uses to display indexes.