Here’s one hidden and confusing restriction about SA top-up: cash top-up to SA can’t be withdrawn at all, even after 55, even if you hit BRS and pledge your property. See https://www.cpf.gov.sg/Members/Schemes/schemes/retirement/retirement-sum-topping-up-scheme and the diagram in https://www.cpf.gov.sg/Assets/members/Documents/Illustration_of_topup_monies_in_RA.pdf

“Cash top-up” means from outside CPF, such as bank accounts.

The liquidity-smart route to SA top-up is

• $100k housing-refund from bank account to OA, assuming your nett OA-usage for housing is below $100k
• $100k transfer from OA to SA, assuming your SA is below FRS

This way, you effectively move money from bank account into SA to earn the very lucrative 4% compound interest, and you still can withdraw it after 55. I confirmed this with CPF hotline.

Q: how much, if any, should we transfer from OA to SA, knowing it might become somewhat “free” after 55? See this blogpost

— https://www.bereadywithcpf.gov.sg/articles/how-a-singaporean-accumulated-cpf-full-retirement-sum-in-his-special-account-by-the-age-of-34/ is a personal story featured on CPF website, where a 34-year old SG national did a lot of OA->SA transfer and cash top-up.

See also my blogpost on 1m65