Interactive Broker vs FSMOne for newbie who plans to DCA once or twice a month for US ETF, and need an easy-to-learn interface?

FSMOne definitely has an easier to learn interface compared to Interactive Brokers. Since you're not actively trading, Interactive Brokers' inactivity charge would also be a downside for you.

To add on, I think one underrated feature of FSMOne is the ability to transfer out positions without any charge to another broker. This allows FSMOne users to change brokers without any cost (although it does take quite a bit of time to get the positions through). Other popular brokers such as SAXO, Standard Chartered and Tiger all have exit fees that range from USD 39 per counter to USD 100 per counter!​​​