I understand your plight. I have been using HomePlug since the first generation AV200 then upgrade to AV600 until now using AV1300 also get less than 200Mbps.
AX5 - 5Ghz can go up to 1200 Mbps, mesh will cut that in half coz don't have dedicated backhaul. Left 600Mbps then WiFi strength will further cut it down.
Better get another AX3600 or AX6.
To put that into perspective, I currently have 2 x AX3600 with theoretical 5Ghz speed of 2402 Mbps.
Main router ground floor and child router upstairs. Straight line distance around 6-7meters.
Upstairs laptop distance to child router around 6-7meters.
I get this speed on a good day with Unifi 800Mbps.


I was quite disappointed with the result. I was hoping I can get around 600Mpbs download upstairs as that's where i spend most of my time in home office.
If really want best speed need to get MESH router with dedicated Haul but that also I don't think can get 800Mbps.
Sometimes think back, spent so much money after all these years. might as well just pull one Ethernet Cable lagi cheap.

Try to pull Ethernet cable to connect both of the ax3600... will it be better ?