As for my use, it is stable.
If you're happy with the stock firmware, I mean, if it serves well, then you can stick with the stock firmware bro. If the stock is honda, then the merlin is bmw.

The answer is yes. You can use the wireless scheduler to do this.

Go to wireless > professional > Enable wireless scheduler <<Click on time scheduling>>

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Then drag/select the time you wish the wireless band (let it be 2.4GHz or 5GHz) to be automatically turned off during the configured time. And yes, it works well.

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Yes. and as bro blacktubi quoted, asus-merlin allows to customize your router according to your own preferences. It also supports scripting and diversion & pixelserv (router level adblocking). The stock firmware is good. But merlin offers you over the edge to do anything you want with your router. I am quoting, "Merlin tend to fix and release security patches earlier than ASUS's engineer does".