Additional downpayment for HDB purchase?

I would advise you to calculate the actual interest saved versus the opportunity cost of putting more cash upfront into your downpayment.

Given the housing loan interest rates now, it might make more financial sense to take the additional cash and invest it to generate higher potential returns then use the returns to make adhoc loan re-payments later?

There is the option to get longer lock in periods to secure the loan interest rate, if there is a concern.

Just take a lower loan amount? That will help lower the interest. Under the financial planning for HDB, you can input the amount you want to pay by cash savings (apart from CPF). https://homes.hdb.gov.sg/home/calculator/paymen...

The deposit (or downpayment) is given to the seller (if resale) - I believe you can do so or direct pay HDB (safer).