Is my ETF portfolio diversified enough with STI ETF, CQQQ, ARKK & QQQ? Is it a good idea to hold both ARKK & QQQ as there are some overlaps?

Without the percentages, it is hard to make an assessment. But assuming equal weightage, I think it's not a bad split honestly.

ARKK and QQQ while there are overlaps, I don't think the overlaps are that big anyways. The main overlap is TSLA but TSLA is only a small portion of QQQ anyways so it shouldn't be an issue. The next 3 biggest holdings of ARKK: Roku, Crispr, Teledoc and Square do not appear in QQQ anyways.

CQQQ gives exposure to China which is good.

Regarding the question about dividends, yes, I do think that selecting certain REITs will give better dividend returns than the STI ETF. Although that being said, you will need to know what you're looking for and how to select a good REIT in order to achieve the desired result.