RAX80 vs R9000 vs Whatever

I would choose the R7800. I have a 1 Gbps down and 50 Mbps up ISP service using a R7800. Using Speedtest.net to the local ISP at 6:00 am, I get 920 Mbps down using wired Ethernet and 650 Mbps down on 5 GHz, 80 MHz Channel, 10 ft line-of-sight to R7800 with an iPhone 8 and a Dell XPS13 (2017). While there are some firmware bugs (Netgear has a very checkered record of Q&A as to their firmware updates the last few years in my opinion), the router is very stable on firmware v 1.0.2.52 and  v 1.0.2.62 (most recent).  I have a two-story, frame-brick house, and 5GHz Wi-Fi coverage is excellent in all parts of the house. At the worst location in the house, I get 170 Mbps down on 5 GHz.

There is only one client device for 802.11ax currently shipping, the Samsung Galazy S10. While most new versions of Wi-Fi have also resulted in better performance for legacy devices, I would wait for the "wave 2" devices that will show up in 18 months or so.  By that time there will also be client devices that can take advantage of 802.11ax's features. The 802.11ax standard is still in draft  and will not be formally approved until later this year at the earliest. For the most part,  however, these draft devices have worked out pretty well looking back at the history of the Wi-Fi generations. Finally, the target market for 802.11ax is large venues with many devices like large classrooms and football stadiums. I would let the four NFL stadiums that are upgrading this year to 802.11ax work the bugs out :-)

The EX8000 range extender looks like a good match for the R7800. There is a review on SmallNetBuilder.com of this combination. I have had good luck with Moca 2.0 bonded devices to get 1 Gbps Ethernet between downstairs and upstairs in my house using the cable TV infrastructure in the house.

I personally think the R7800 is a better choice than the R9000 for most people. Both devices have the same Qualcomm 2.4 and 5 GHz radio chips.