(re wifi mode, I've tried all the possible settings. currently, mac shows connection as ax.)
I tried a few more setups this evening:
In order to eliminate the macbook pro's wifi radio, I added the GL-MT3000 I purchased at the same time to the mix.
(laptop wifi off during all tests, using ookla's speedtest app for all speeds)
Laptop -> WAN = ~900mbps down, ~800mbps up
Laptop -> ethernet to GL-MT3000 -> WAN = ~900mbps down, ~800mbps up
Laptop -> ethernet to GL-MT6000 -> WAN = ~900mbps down, ~800mbps up
Laptop -> ethernet to GL-MT3000 -> ethernet to GL-MT6000 -> WAN = ~900mbps down, ~800mbps up
Laptop -> ethernet to GL-MT3000 -> 5GHz Wi-Fi (11a/n/ac/an/ax 160MHz, DFS channel) -> GL-MT6000 -> WAN = ~320Mbps down, ~580Mpbs up
Laptop -> ethernet to GL-MT3000 -> 5GHz Wi-Fi (11a/n/ac 80MHz, auto channel) -> GL-MT6000 -> WAN = ~300Mbps down, ~500Mpbs up
Laptop -> ethernet to GL-MT3000 -> 5GHz Wi-Fi (11ac/ax 80MHz, auto channel) -> GL-MT6000 -> WAN = ~300Mbps down, ~600Mpbs up
Laptop -> ethernet to GL-MT3000 -> 5GHz Wi-Fi (11a/n/ac 40MHz, auto channel) -> GL-MT6000 -> WAN = ~220Mbps down, ~170Mpbs up
I figured the two routers should be able to hit max speeds between each other, but regardless of wifi settings it was always about 2x as fast to upload as download (until I hit 40MHz in the last test)