First of all, I must say you have conducted a thorough research and the impressive SRS you attached clearly shows that. Though, I feel there are some points missing:
1. Placing orders through APIs may have latency of some seconds (we may face price fluctuations - real vs calculated)
2. Zerodha does not provide sandbox/mocks to test the APIs or debug our code (I have not checked other providers)
3. After placing the order, if there is any issue (broker server not responding, internet issue or any other technical issue), there is no configuration of SL to prevent huge losses. Broker servers work OK most of the time, but when they fail, can result in huge loss (I have experienced this).
Approach:
* The best way to build desktop application for this purpose, that can start and respond fast, is to use Python Tkinter.
* Using multi-thread architecture (different threads for different brokers to fetch data, place order, etc.) will result in faster execution.
Let's connect and discuss this project in-depth including features, price and timeline.