With your income, I think it is best you take a loan of $500k the most to be safe as interest rates will go up. In this case, you should sell your flat and use the proceeds as downpayment for a condo. Let's say you sell your flat for $500k. You then buy a 3 bedroom $1.3m condo. You need to use the HDB flat sales proceed of $500k plus $300k cash savings and CPF for a $800k downpayment for the $1.3m condo. Loan will be $500k.

If you don't sell your flat, you need to have $800k of cash savings and CPF for the downpayment. How much cash and other assets do you have? What's your total net worth now?