Some progress here. I think there’s actually not a limitation of 20Mhz when in repeater mode.
I did the following: setup the router with ethernet on the WAN side, and then all the wifi settings become unlocked. Setup wifi to the max of 160Mhz channels, and indeed when connecting from an ax client it connects at 2x2 with 80Mhz channels.
After this, I setup repeater mode again, and now the 80Mhz channel setting is preserved. Seeing 300mbps on repeater mode now which is fantastic.