I am in for a simple 2d traffic depiction on the EHSI, too. (I wouldnt call it tcas...)
Forgive me my ignorance, but aren't you guys using the default x-plane weather radar, of course with multiple layers on top? Maybe it would be "relative" simple to add the default traffic symbols to that layer and customize the texture of the traffic symbols? I personally wouldn't care much about the various different symbols and "modes" (filled diamond, dot, squares etc.) but just to have a very basic indication of surrounding traffic maybe with a relative altitude.
I'm just thinking out loud, you probably had this idea already years ago
Greetings
Jonas