I want to invest for the first time and because I am beginner with little knowledge of the stock/finance market, is it wise for me to start with POSB Invest-Saver with $200 for a start?

I started that way too, with DBS Vickers, but I find the limited options a little "boring" for my taste. I think it's best to be diversified, yet like you, I am a relative newbie to investing (approx. Less than 1 year). So what i did instead was I opted for Robo-Advisors cos of its Low cost and it is automatically diversified, based on your risk.

I chose stashaway due to its very easy friendly interface and it's low costs. U can start as low as $100 and no fixed amount needed every month, but I recommend DCA.

Well it's a good start, but it may not be the most diversified option. Good to have global ETFs in your portfolio as well and you can do so by Robo advisers for a fuss free way.

Yes, it would be a good start as you will be investing in the top 30 companies in singapore. Meanwhile you can research about the other investment products out in the market.

Ideas:

https://seedly.sg/questions/what-is-your-general-investing-philosophy-strategy

I hope you had not invest in STI, your returns would be petty at best so far

Yes. Opens your first exposure to the market and subsequently you will venture more in the market. Which what happened to me Too! :)

Yes, it is a good start to use a regular savings plan to do dollar cost averaging like POSB investsaver to purchase the Nikko AM STI ETF

Can consider using StashAway to expose yourself to the US market as well