I personally don't recommend the ath9k cards. There are a handful of routers they do not work with. You'll have to disable QoS to stop the packet drops.
N is not modern in any sense of the word. I think 6 is more used then you would think. All ISP I know are giving out 6 access points and have for awhile.
I install residential and business internet for a living and I have yet to encounter a single AX AP operating in the wild (yes I check every time, and yes my devices support it). And our own routers only do N.