All in all, a convoy and a singleton are the same thing. A singleton is implemented using a convoy.
There is no reason you need any type of batch or timeout. That’s just a more common scenario.
For a singleton, you’d just have a loop and have it never end. Loop until 1 = 0 or something.
You can use some like Received Port Name rather then message type, assuming everything is on the same port. Or you can create a custom pipeline that added some unique context property. The correlation can be on any promoted property that would unique to your group of messages. You could even use something like PO Exists.
Hope this helps.