I understand you wish to have some flexibility in cash, but I highly recommend setting some of it aside to make sure you are provided for FOR LIFE, so that no matter how much you screwed up, you will still get a nice payout every month for the rest of your life. The best way to ensure that, of course, is to make the most use of CPF Life from age 65, maximizing your SA at age 55, and plan your drawdown carefully between the age of 55 and 65.
So what you should so is to Shield your SA 1 month before the age of 55 (you can look up SA shielding through Google, there's a couple of threads here on this topic plus a couple of other blogs).
At age 55, your RA will be formed. $50k from OA + $40k from SA. Now immediately use cash to top up at least $91k to at least reach the FRS of $181k. Now you can drop the SA shield and have $160k in SA earning 4% interest. After which, you should then use $90.5k of cash to top your RA up to the ERS amount of $271.5k, leaving you with about $18.5 of spare cash.
You can withdraw as much money as you like from your SA as you like. $160k in SA allows you to withdraw about $1620 per month from age 55 to 65, and your SA will be depleted when you each $0, after which you can apply to start your CPF Life payout. The standard plan will pay you out ~$2200 per month for the rest of your life, or you can choose basic or escalating instead.
This is of course, not taking your maturing policies into account. You can choose to cash those out and live on cash spending instead while leaving your SA to grow. Once your cash runs out and you start spending on SA instead, your SA may even be able to last longer, perhaps up to age 70 in which case you can delay your CPF Life payout to start at age 70 instead for bigger monthly payouts.