Here's what I'd do....
(a) Make sure your household is appropriately insured, in a cost efficient way, against potential calamities. That usually means the "big 3": simple term life insurance, a decent Integrated Shield medical insurance policy (an "as charged" B1 ward policy is the economic choice), and disability income insurance.
(b) Retire any high cost debt, such as credit card debt. (Your HDB loan at 2.6% is not high cost debt.)
(c) Accumulate an emergency reserve fund that will sustain the household for at least 6 months, preferably 12, if your income from work were to cease. Cash in the form of your ordinary bank deposit (~2 months) plus a Singapore Savings Bond (~4+ months) is the preferred formula since cash can be used for anything, although your CPF Ordinary Account is a second best choice for the portion that would service your HDB loan.
(d) After taking care of the first three imperatives, to save for retirement I'd put your first 14,000 cash dollars into CPF Special Account top-ups: $7,000 into your SA, and $7,000 into your non-working spouse's SA. I'm assuming that would mean you qualify for $14,000 of tax relief, and this tax relief is why you should use cash for your top-ups, first. If you can only afford $3,000 per spouse ($6,000 total), OK, do that. (Although probably just put $6,000 into your spouse's SA in that case. Prioritize contributions to the CPF SA of the spouse that still qualifies for bonus interest, which is probably your non-working spouse's SA.)
Read the tax rules carefully to make sure you understand whether you qualify for tax relief, but take maximum advantage of the tax relief if you can.
(e) OA to SA conversions can then optionally come after satisfying the above steps.
(f) If you still want/can save more, then you can explore dollar cost averaging into a low cost, well diversified index fund. I do not recommend using OA or SA for this purpose since the CPF Investment Scheme choices are not attractive.
