R7800 is not good enough for your use case. You can get 750-800 Mbit/s out of it, using tricks like irqbalance and flow offloading. Anything more requires the use of custom builds with support for the "NSS" engine. It is conceptually similar to hardware flow offloading, but uses a different interface.
So - sorry to say that, but the device that you want doesn't exist. Linksys mvebu-based devices can route gigabit easily, but have a bad and unfixable WiFi driver (known incompatibilities with ESP8266 based devices, strange way of setting the country code, and no WPA3). R7800 is a bit too slow. ESPRESSObin v7 (or maybe ESPRESSObin Ultra) has a fast CPU but only 2 external antennas.
My recommendation would be an x86_64 box, with a recent MediaTek adapter added (if you are OK with a single 5 GHz band, or can install a separate 2.4 GHz adapter). This Mediatek card looks good on paper, but I have not tested it personally: https://www.asiarf.com/shop/hot-sales/wifi6-4t4r-dual-bands-selectable-mpcie-card-ieee802-11ax-ac-a-b-g-n-2-4g-5ghz-aw7915-np1/ . Do not buy Intel WiFi cards, they cannot function as access points in the 5 GHz band.