thanks! We did some tests this morning. And we are able to connect to S3 AWS via corporate proxy if we just create the S3 repository via Kibana Console.
From there we can create the Snapshot Policies and both work in parallel, the ECE snapshot policy includes all indices. And the "Kibana Console" repo / policy only the system indices. See screenshot.
This means we can use ECE snapshot for customers that do not have an issue that their data is snapshot to S3. For users that don't care about data backup we can use the ES API S3 repo for system indices snapshot only. And for customers that need local DC backup, we can use ES API (and yaml) config for a local filesystem backup solution.
P.S. via the Kibana GUI we can select indices for the repo created via ES API.