I gave up on aimesh. Configured 1x XT8 in repeater mode. Amazingly, getting 7xxmbps internal pc to pc testing, 6xxMbps with aimesh. Will see if I can get the other XT8s up in the same way.
Edit. So...replaced the XT8s with XT12s. AXE16000 is running firmware FW_GT_AXE16000_300438650168. XT12s, had to upgrade firmware to FW_ZENWIFI_XT12_300438650350. Both of these are not the latest firmware but they did allow me to get it all running in aimesh. Processs I used.
AXE16000. Factory reset via WPS button method. Login. Restore + checkbox initialize. Login. Wait 15mins. Restart via power off switch.
XT12. Factory reset via WPS button method. Login. Restore + initialize. Wait 5mins. Restart via power off switch. Connect via Ethernet cable. Set as aimesh node. Switch to AXE16000 > add aimesh node.
Note. I cable connected the routers initially. Once aimesh node is added. Wait 10mins. Restart XT12 via switch button on/off. Let aimesh sync up. Leave 10mins. Disconnect cable from XT12 to AXE16. Wait until it syncs via WiFi and backhaul is 5Ghz-2, then wait 10mins. Then switch off XT12 using switch. Power on after 20-30 seconds. Let it sync back. Wait 5 mins on sync and backhaul is 5-2 band. Switch off/move to final location.
I have the AXE1600 + aimesh wireless 2x XT12. Rebooted nodes numerous times, optimize, etc. All good so far.
What I learnt is that waiting...its actually required. Numerous times I saw the XT12 lose sync as is went from 2.4Ghz backhaul to 5-2Ghz. Switching from ethernet to wireless is causes the to re-synch several times. Firmware is offical Asus. I tried Merlin, but could not even find the aimesh node option on factory restore on XT12. If I was running 1x router, I'd go Merlin every single time but, I am fine with it.
Only odd thing is that the dual-wan is not flipping back once the primary link comes back on line, not too bothered. Just super happy I got the whole network so far, 2nd XT is first time solid