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?

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.

If you have 500k excluding CPF, you are good to stay at your current flat without downsizing. However, if you want to get more cash and dont mind a moving to a smaller flat, its definitely a wise choice.

Keep your RA at FRS. Dont go for BRS. It will affect your monthly income stream afterwards.

For your 500k, I hope its in stable blue chip stocks that give stable dividends. At 4% p.a. you will get a passive income of 20k a year which is around $1600. Together will your FRS payout and the excess money from downsizing, it should get you comfortable at 6k/mth.