https://www.asuswrt-merlin.net/about
It is enhanced ASUS firmware. It starts with the base and adds features, optimizations and enhancements. Installing is easy and removing is easy as well. Unlike many routers ASUS even has a failsafe mode which the router goes into if a flash goes wrong due to a power failure which allows you to reflash.
I do recomend you watch this though: https://www.youtube.com/watch?v=2Dnic-DU-GE
The IQRouter is another option you should consider. This is a review of an older model. There are tests you can run which will tell you if you can benefit from it and it is comparable to the ASUS 1900.
I will add this to my selections. I checked out the Amazon page and this is something I will definitely put near the top of the list. I like what I read.