My suggestion is to do dollar cost averging every month and average down on any stocks which you think may have potential for a comeback

“Far more money has been lost by investors preparing for corrections, or trying to anticipate corrections, than has been lost in corrections themselves.”

— Peter Lynch

Since your biggest concern is a market downturn, you may want to do dollar cost averaging of about $200 to $300 every month. This is because time in the market is better than timing the market. You never know when is the best time to enter the market. Just my humble opinion.

You are asking an incomplete question. I guess I will throw back the questions back to u.
Do u think the valuation is high now? Price and value are different. Price is what you pay. Value is what u get. I believe that if u look hard enough, there are stocks that are undervalued.
You mentioned that the current market valuation is too high. You seem to measure it from STI, but does STI show you the full picture of Singapore economic climate?

You mention that you want to start to build a dividend portfolio. So, I infer that you want to take a passive - active approach. You buy a stock to collect dividend. This means that you will be keeping the stock for more than 5 years.
So is there a need to panic over a market downturn? Isn't it a good time to buy the same good stocks at a cheaper rate?

Your homework then be defining a good stock so that you can jump right in when the price is right.
Thus, I feel that your concern is unjustified.

I believe the ans can be found in Seedly, yourself and/or the dividend/ investment workshop