Steve,
Reading through your description, this is exactly how my dual DCS / Legacy system is set up, except I do not have a need for the serial to USB converter.
If you have a need to run anything conventional, I would strongly recommend you add a Lionel Legacy Powermaster. It provides substantially improved variable voltage control over that offered by the TIU hardware. I have my Powermaster programmed as an engine labelled "Powermaster" with TMCC ID #1. I then use the speed dial in the DCS app to control voltage.