I look at your situation a bit differently.
1) Since you only paid 240k and not the currrent valuation of 400k.
Your actual rental yeild works out to be $24k/$
240k x 100= 10%
Even if rental falls, you will still get a pretty decent yield.
2) The profit you make by selling now is 160k. Worse case situaition is that there is an oversupply in punggol flats and the price drops back to 240k forgoing your gains. Assuming you can get tenants, this loss can be offset by renting out your flat for 6 plus years.
After 6 plus years you can still get rental income of 24k per year so your upside is higher than the 160k profit that you will have by cashing out now.
3) Since you bought your HDB at a low price, you are in a strong position to grow your wealth. It is unlikely that the price of punggol will drop back to 240k so your downside risk is very small and any potential loss can be coverd by your rental.
Becuase you have bought a private property while still owning a hdb flat, the other option of
relocating or downgrading your hdb flat is out. The next best course of action is to hold your hdb flat for rental due to your low purchase price and high rental yield provided you have the cash flow and meet the cpf minimul sum or can pay by cash.
Also do the maths and max out your hdb punggol HDB loan to 30 years. Dun worry about the interest you will be paying, your rental more than makes up for it. If you do it right you will have 2 fully paid up houses down the road and can look to buy a third.