This is one of several ASUS routers (RT-AC66, RT-AC3200) for which you should use the newest ASUS firmware for security reasons.
To be safe and relatively simple to operate you will have to give up Merlin enhancements to ASUS firmware for these routers. ASUS is continuing support for security patches on several once popular modems long after development of new features and fixes to existing features and functions has been abandon by ASUS and Merlin.
While Merlin usually creates a much more polished and flexible product than ASUS factory releases, he can only concentrate such polish on the current mainstream code boilerplate used by ASUS for newer routers. Such are the limits of a one person operation and the current ASUS boilerplate mainstream code has moved on from what is used on RT-AC66 and RT-AC3200.
If you need features beyond the official ASUS firmware, you will have to give up simple and try out DD-WRT or OpenWRT. Security patches will probably happen but schedule and guarantees are uncertain as with any large but less well sponsored Open Source project (neither DD_WRT nor OpenWRT have anything like Ubuntu level of corporate aid and sponsorship). As always in these situations assessment of security falls to the end user and its somewhat expected that end users either contribute to coding solutions to any outstanding issues or hold their peace until it happens otherwise. Good Luck as it may take weeks or months to master configuration of either of these firmware approaches if its your first time. However, that time may fall to a few days or weeks if your needs fall very close to one of the available pre-configured setups for these routers that can be found in tutorials. All told these can be great solutions for those with patience and will to bear with a few bumps. Also be aware that much of the bumpiness is there or worse in factory firmware and configurations but you are much more aware of the bumps and issues because you are actually allowed and forced into knowing details that the factory (and even Merlin) firmware conceals.