I tried out the rc2-7 build on the RT-AX58U and also found that it worked pretty OK. I did not run it for weeks and weeks or anything because it doesn't have the @RMerlin features that I depend on. I just kicked the tires for part of a day and the main features I use seemed to work. I did a clean flash.
I tried out the rc2-7 build on the RT-AC88U and couldn't add any of my Lyra's (factory reset first).
Maybe below is right and AC88U has more bugs:
I think you're beta tester number 3, maybe 4, to say they cannot get the system working with an RT AC88U router and beta firmware. I'm back to being convinced there is a device specific bug to that model. The "disable Ethernet backhaul" lured me into a false sense of hope, as that did get me farther down the road towards a working system, but that was only part of the problem.