40 yrs old single, saving for retirement. I've only invested in STI ETF using OA. Thinking of selling it soon and switch my OA to Endowus instead, is that a good idea and which fund to buy first?

Thats definitely a much better option than STI, super terrible index

Anything is better than STI ETF.
It depend on your risk appetite: x%equity x%bond

Also thing to consider, your current $$$. (Assume retire at 65?) 25 years in the future with the type of portfolio u select (e.g. 60%E, 40%B) will the projected amount sufficient for your retirement? If not u need to take up more risk.

OPINON.
If i going long term, i will be aggressive 100%E