I’m thinking of starting to invest $1.5k in STI ETF and $500 in S&P next month onwards using DCA. Is it wise considering that it’s bearish for STI ETF and yet all time high for S&P ?

How do you intend to buy into the S&P through a DCA method? I'll assume you are referring the S27 SGX counter? There is no automatic way of buying into S27 and the minimum share lot is 10, which works out to be US$2650~ per transaction based on today's prices.

If you are buying to SPY, you are looking at perhaps 1 share a month based on your $500 contribution. Which doesn't make sense, as the brokerage fees will be quite high.

Unless you are thinking of buying into the Infinity S&P500 fund issued by LionGlobal, in which case I'll suggest you stay far away. The fees are rather high which will eat a big chunk of your paper gains over time.

Nobody knows where the market is going, that's why a DCA strategy is good because it gives you a plan to buy consistently regardless of whether the market is up or down. Personally, I would overweight more on the global indexes rather than STI ETF.