Clarify: I would touch CPF interest. Not RA. Perhaps SA if I had to.
But you may be right that it makes more sense to sell MBH first.
That's not necessarily what I suggested. (See below.)
If you're digging more than lightly into CPF at age 55+ then you're withdrawing from SA first, the 4% interest earning part. Conceivably you could avoid that past $40,000 by raising a Special Account "shield" (CPF Investment Scheme-SA), but that's a hassle.
I'm not sure why the need to pick the "too high" allocation knowing that stocks are much lower. I would sell from MBH and do my best not to touch IWDA or ES3 at all, even if I have to sell all my MBH.
Stocks are much lower, but you still want to rebalance your portfolio periodically (once or twice a year is enough) to keep your portfolio aligned with your desired target allocation. If you need to raise some funds for day to day living expenses, you can still apply that rebalancing principle and start with that first. And that'll work well precisely because stocks are probably "low," meaning that you'd probably favor a bond fund sale first since bonds have held up somewhat better. If/when stocks rise back up (and perhaps beyond) their prior level, the next rebalance will reduce the stocks.
If you're in perfect balance to your target allocation then you'd draw evenly from stock and bond funds.
Hence the need to plan? If you already have it in OA, that's the plan. If you don't, presumably, you need a plan.
My point is that most CPF contributions are compulsory, and there are many people -- not all certainly, but many -- who have no particular problem piling up CPF dollars. It "just happens." In that rather common (but no, not universal) situation, you shouldn't have to worry about raising cash for an ERS top up at age 55. Of course this'll depend on how much and how quickly you're draining OA for mortgage payments, how much you're topping up your SA at a $7,000/year clip for tax relief, how much you've topped up MA for tax relief, how early and how much you're performing OA to SA transfers, whether your employer participates in the AMCS, your earning level (including variable pay), and several other factors. But it is rather common to be relatively flush with CPF dollars at age 55, flush enough to be able to push a RA up to the ERS.