Just completed National Service and have a total of $18k in savings that I'm looking to grow. What would be the best investment approach for long-term returns?

IMHO, SSBs yields are too low for any real returns that can even beat inflation in the coming years and keep you ahead of the herd over time. If you are a "lazy investor", just invest generously in the S&P. However, if you are looking for more flavour in your portfolio, pick out some mega cap stocks to hold over the long term that have great business and fundamentals. FAANG companies are just one example of them.

It depends on your risk appetite. SSBs are pretty safe compared to stocks. In terms of ratio also same thing. If you are capable in dealing with stocks volatility which means you wont panic sell when prices start dropping, you can go 100% stocks. Make sure to have some emergency fund usually 6 months expenses. But since you not working now, maybe put like $2000-$4000 in SSBs and $1000 in the bank for any emergency. SSBs withdrawal takes some time so you need some money in the bank. Another thing is that if you take student loan for university, i would suggest using the money to pay it because debts interest are guaranteed unlike stocks returns. Robo-advisors focus on US and STI ETF focus on Singapore depends on which you prefer although STI ETF are not really that good. This is my opinion, you can tweak it if you prefer something else. You have to remember if you invest in stocks, you might lose money and dont expect it to only go up and dont go down.