Should I get resale flat, or bto and rent (for approx 3 years till the flat is ready), if the full amount is still cheaper than a resale?

If you value time more than money, get a resale.

If you value money more than time, get a BTO.

I would advise calculating the exact numerical savings you get if you need to decide between BTO and resale, including the rental which you need to fork over.

The PPHS scheme can be considered as well, there is a wonderful article here:

https://blog.seedly.sg/parenthood-provisional-h...

Depends on your plans. Getting a BTO at a discounted price $150k - $200k below market value would be ideal, but due to the long waiting period on top of the delay from covid, it may not suit you later on.

Especially if you are considering kids, you may be looking to move to live near good schools. In this case, 5 years of construction + 5 years of minimum occupation period would be about 10 years before you can relocate.

As for rental, depends on whether you and your spouse are willing to stay at each others places to save on costs. Otherwise, you can also calculate how much it would cost to rent for the next 3 years and weigh it against how much you'd expect to sell the property in the future.