Hi Shaun,
Please be aware there is no requirement to acknowledge the message in the callback. So you don’t need to go the lengths of persisting the message in the callback just so you can safely acknowledge it.
You must acknowledge the message before you Dispose the flow is the only requirement. So you could simply put all your ‘process message’ logic in a thread that is reading messages from your own defined message queue. The acknowledge the message from that thread.
It may also be possible to use an executor service or pool but I would avoid starting a new thread per message.
Ragnar