WAX206 Wireless AX or AC Upload Speed Disparity

Could you test limiting your 5 GHz Wi-Fi to 11ac mode?

The speed problems should only affect 11ax mode. I think I see the same issue:

Big forum thread about this issue: [Solved] 802.11ax worse than 802.11ac with mt76 driver?

The fix of dropping 160 MHz channel support that Mediatek implemented was rejected by the community, because some green field users use 160 MHz channels in areas with no or few other 5 GHz network interference:

The solutions that have been found to fix 802.11ax 80 MHz channel bandwidth so far are:

  1. Remove IEEE80211_VHT_CAP_SUPP_CHAN_WIDTH_160MHZ | in mt7915/init.c as does this pull-request to the mt76 wifi: mt76: mt7915: disable VHT CAP SUPP CHAN WIDTH 160Mhz for mt7915. This will remove 160 MHz VHT (802.11ac) bandwidth capabilities, which may be undesired.
    There is also [MAC80211][mt76][remove mt7915 BW160 support]
  2. Some report switching to 160 MHz bandwidth solved their issue. To make this work, your client devices may have to support 160 MHz bandwidth. Not sure.
    Maybe there are more solutions, which have not been published by Mediatek yet, but this is what is publicly known at the very moment about this problem.

Also I suggest to avoid sae-mixed mode. Instead better create two SSIDs, one with sae WPA3-only and another with WPA2 CCMP forced for stations that don’t yet support WPA3.