It sounds like you did not try the solution I made… If you don’t have experience in how to use help given, then there is not much to do about it. You will only learn how to work with dynamo if you are able to tweak whatever help you get to some extent by your self. So I will recommend you to give it a second try.
Since you did refer to a previous solution I have made, have I made a solution for what you are asking for. This graph split the ducts in a length driven by the user. It unions the ducts with a fitting of a length driven by the user. For this graph, I added joining the entire network together at the elbows, since that was the problem you showed in your screencast.
You should be able to add more intelligence to the graph, so you can have minimum lengths as well as the default cutting length if you think this is better. However, I would believe that the duct lengths you can buy should be the default length, and then there should be as few cuts as possible since every cut will cost extra to do.