If I retire with about 500k in cash and stocks, and have met the FRS, would it be a good idea to sell my 30yr 4 room hdb and downgrade to a senior 2room flexi, or should I pledge my hdb for BRS?

Wow. how young are you? I wish I am in your shoes

You shouldn't have a problem retiring.

You only pledge your home for a reduced payout because you can seek an investment that has a higher return than the FRS from the free cash that will come from freeing the sum after pledging. The only risk you both might face would be that stocks are subject to volatility as you age, which might not be a great asset class for high quality cashflow if you are depending on it for cash flow later on. If you intend to maintain your standard of living at $6,000 a month after you retire, you should start planning as to how you can achieve that with your current assets.