Having a long timeout should be fine, and certainly as you say you want redelivery if something goes wrong, so you want to only ack after you finish.
The best way to achieve that, IMO, would be to have multiple consumers on the queue (i.e. multiple threads/processes consuming from the same queue). That should be fine as long as there's no particular ordering constraint on your queue contents (i.e. the way there might be if the queue were to contain contents representing Postgres data that involves FK constraints).
This tutorial on the RabbitMQ website provides more info (Python linked, but there should be similar tutorials for other languages): https://www.rabbitmq.com/tutorials/tutorial-two-python.html
Edit in response to comment from OP:
What's your heartbeat set to? If your worker doesn't acknowledge the heartbeat within the set period of time, the server will consider the connection to be dead.
Not sure which language you're using, but for Java you would use the setRequestedHeartbeat method to specify the heartbeat.
The way you implement your workers, it's vital that the heartbeat can still be sent back to the RabbitMQ server. If something blocks the client from sending the heartbeat, the server will kill the connection after the time interval expires.
