I was curious if it is feasible and worth my time/money to build a trading platform using external APIs and libraries or is it easier to use something like TD Ameritrade. In other words, are there any quantifiable upsets, risk mitigation, or arbitrage opportunities associated with building my own trading platform and using my own algorithms to execute buy/sell orders. This may be the wrong group to ask this question.
It is unfortunately absolutely, totally, inconceivable this could ever amount to anything.
Best course of action: first get $20,000 you don't need, consider it a "college fee". Put it in a trading account and learn to trade. Once the money is gone, start over.
With luck you will only have to do this 4 or 5 times until you have a basic feel for markets and trading.
It would literally be easier to say
"I notice Boeing make money building 747s. Could I build one and compete and make lots of money?"
The companies that run algos to arbitrage the market literally deal with technical issues such as the speed of light on their software-hardware systems to eke out fractional wins.
You'd need $20 million annually just to hire a few basic engineers at this level to even vaguely get started in the game.