So let's explore this a bit more....
previously i have the same idea.. return to OA then xfer to SA from OA. but i think it will remove the opportunity for me to top up more using cash into SA if i reached FRS by (OA->SA)..
Once your SA reaches the FRS, further top-ups and conversions are not allowed, correct. Compulsory contributions will continue, though.
However, every full month without money in your SA is lost interest. If you're thinking of the tax relief, that makes sense, take it first, now. A January $7,000 cash top-up is a very good idea. I'd do it within the next week so the funds are credited to your SA in January.
Waiting for each year's $7,000 tax relief top up, though, might or might not make sense. At least, it's not a straightforward calculation. It'll depend in large measure on your tax rate and how much interest your cash can reliably earn outside CPF. It's best to create a simple spreadsheet to figure out whether you should push more than $7,000 of cash in now versus waiting for more tax relief.
the lesser cash i can top up, means lesser i can earn the interest..
Yes, SA (and MA for that matter) earns higher interest than OA or than cash outside of CPF. The faster your SA (and MA) balances zoom up, the more compounded interest you earn. A Singaporean baby with a generous grandparent, for example, who deposits $166,000 into the baby's SA is a very lucky baby.
my idea is to withdraw all the excess when i hit 55.. should be able to accumulate quite a bit with all interests.
Why would you withdraw all the excess when you hit 55? It'll still be earning annually compounded interest, a lot of it. The financially prudent decision is to withdraw CPF funds only when you actually need the funds
and you have tapped out other sources of funds that are earning less interest. If you don't need the money, or if you have other money that isn't working as hard for you, don't withdraw
anything from CPF -- and certainly not a full withdrawal. That'd be financially crazy, wouldn't it?