1024-QAM ? or alternately, which router to buy?

To be fair, you might have the perfect use case of NitroQAM if you have a PC with Broadcom Wifi and no ethernet located three feet away from the router

QoS is mature now thanks primarily to fq_codel, which made my bufferbloat score on Comcast go from F to A. I would not get a router without this type of QoS as it works great to minimize latency when under heavy load. It keeps RAM buffers in routers across the internet from filling up on their way to or from you (which could cause ping measured in multiple seconds) by discarding low priority packets, but has no effect on high-latency wireless links like 4G or Wifi.

As for futureproofing, the folks behind fq_codel have a new ambitious project to minimize latency of Wifi. It would make sense to buy a router with Atheros or Mediatek chipset that can be flashed with open-source firmware later when this project is complete, or to buy a cheap router for now and expect to get another then. The main problem with wifi is its unpredictable ping which can be over 900ms. Rather than trying to fix this the IEEE have been pushing ever higher bandwidth (as it gives nice large marketable numbers) even at the expense of latency.

Compared to the elephant in the room that is latency, the issues helped by MIMO seem minor: multiplexing again improves bandwidth numbers while beamforming improves reception and rejection of interference. If you are close enough the high signal strength overwhelms any interference, so I prefer to use many cheap APs!

A tri-band router is just two 5GHz radios in the same box as the usual 2.4GHz one, mostly to generate a large number to print on the package. Theoretically you could have two 5GHz devices connect, each to its own radio on the router so each gets full bandwidth--but there is no way to guarantee they won't both just connect to the same radio, though MU-MIMO would help. I would much rather have a dual-band router plus a dual-band AP, as then you'd get an extra 2.4GHz radio out of the deal to put on a different channel, plus you could space them apart because each device is far more likely to connect to the closer radio.

Keep in mind that just about any router can be repurposed into an AP if you get a newer, fancier router later. Ones that can take 3rd party firmware are even more flexible so can be used as a bridge or repeater, or even as a client to connect a wired network to the coffee shop's free Wifi, for example.