Any thoughts on monthly buying an ETF versus buying stocks of the ETF's top 5 or 10 holdings in terms of cost efficiency and long term performance? For eg for QQQ or QQQM.

Generally, if you think you cannot beat the return of an index, it is better to invest in index, at least this is to ensure your return in on par with the index. In fact, majority of the investors cannot beat index such as S&P 500.

Picking the top 5-10 holdings will likely to give u better returns in the shorter run (still at least 3 years holding period)

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However, if you look at history, the top holdings of indices always change. Warren Buffett pointed this out before in one of the recent shareholders' meeting. The top 20 holdings before 2000 was completely different from the ones now and this is not a new thing.

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Ultimately, if u want to go into picking ur own stocks, it is imortant you know what u are doing and how to continuously evaluate them