Can I do DCA (dollar-cost averaging) on REITs ETF instead of investing into 2 or more REITs?

Yes you can. By investing in a REITs ETF, you are in fact simplifying your investment by own 1 share (the REITs ETF) that covers a basket of REITs. A REIT's ETF can own as many REITs they can. You buy 1 REIT ETF but you actually owns everything. However, if you were to do it yourself, you will end up monitoring and buying that many ETF which might not be substainable.

In terms of fees, REITs ETF has a mangement fee within the fund itself on top of the one monthly transaction fee. However, if you were to invest in more than 2 REITs, you will need to mutiply your transaction fee by the numbers of REITs you buy. You will also end up paying more if you invest more REITs.

Yes you can. but the cost will be higher. there is a management fee with REITs ETF