OST T3 has the StuG, which completely changes the equation. If you go for the AI-focused Ostwind, you can get a 90 FU StuG to back it up.
If you go for a Centaur, your AT options are the 110 FU Cromwell or the 155 FU Firefly.
Neither are good options. The Firefly is terrible as a lone AT piece, and the Cromwell at best matches medium armour.
The only thing keeping the Centaur a realistic option is its huge AI advantage over the Cromwell. If you narrow that gap too much, you kill off the Centaur completely.
Your entire argument is based on the FF being bad as a lone AT, despite the 6 pounder being one of the best AT guns and readily available.