I time or count based loop is very prone to Zombies. That’s that these messages are. Messages in the Completed with discarded messages state are very easy to, well, terminate. If you end the Orchestration, they just go away.
I think a better approach would be to write all the messages to a SQL table when they arrive. Then, use a Stored Procedure to extract messages out in groups of 500. I think that would be a safer way to go.
Hope this helps.