You might take a look at the Glacern EM90-750B with APGT1135 inserts. It's not exactly what you were looking for, but a TTS ring added to the 3/4 shaft gets one a "TTS compatible", and it's one piece. The shaft is about twice the circular area of a 12mm tool (that is, twice as stiff). Can mill to 3" depth. I've got one, like it. My only problem with it is that it's another bloody insert to stock.
I've looked at the TTS thread-on cutters. Seem to be Mitsubishi sourced, and as you note, expensive unless used for production.