I'm 28 and have so far invested 21k in both AIA and Prudential 's cpf investments. What other investments should I do to build my cpf funds?

Have u considered topping your special account?

You will earned a guaranteed rate of 4%. Since you are 28. For the first $60k in your CPF combined balance (with only up to $20k max from OA), the monies will earn an extra 1%. There since you are still young, u will not have hit the limit yet. Thus it is worthwhile to topup to earn effective 5% return. Risk-free, regardless of market conditions.

In additional u will receive tax relief for the topped up amount. Eg, your taxable income is $40k, for every dollar u earn above 40k is taxed at 7%. Therefore, if u topped up $1000 into your special account, u get $1000 tax relief, u saved 7% of $1000 = $70 tax money. Effectively have gotten 7% return on your money, in additional u will earn 4% interest yearly guaranteed.

You should look at the where you're invested in and your portfolio.

ILP sub funds are feeder funds are more expensive than investing in direct funds through an investment platform/brokerage like iFast.

You may want to re look into your funds and if your portfolio is giving you an equal global exposure with an asset allocation percentage relative to your risk profile with quarterly rebalances.