What is the best trading platform to use if I’m a day trader, trading estimate $300k transaction per day of US stocks? Looking for one that offer low commission yet low risk and safeguard my funds?

If you making very large trades often, you may not want to go for 0 commission brokers such as TD Ameritrade and Firstrade because they sell order flow. They receive payments for routing your order through a wholesaler and this could mean that your trades may not get executed at the best price possible.

Instead, go for brokers such as Interactive Brokers that do not take payments for selling order flow. Because you are making very large trades, the amount saved through price improvement could outweigh the cost of commissions. IBKR even boasts that they have the "best price execution", but you should do your due dilligence by checking the brokers' SEC Rule 606 disclosure reports which detail such behaviors.

As for the security of your funds, most brokerages should be fine whether it is local or overseas. After all, you are trading on the US markets, where the majority of shares are held under custodians. Brokers are required to segregate their customers' accounts from their own and hence your investments will still be protected even if the broker folds.

I would recommend Interactive Brokers. Interactive Brokers is a professional platform specifically geared towards traders. The commissions are low at ~USD 1, or even as low as USD 0.35 (using tiered pricing). FX rates are great and Interactive Brokers have been around for a long time and are a well established broker. Unlike TD which is also a commonly recommended broker, Interactive Broker does not sell order flow, which may be significant for a daytrader. All the best and happy trading!