thanks @olci!
First run after compiling main branch with your config:
three other machines were connected and using network too so I guess it can go even futher.
Now I hit other problem thermal throttling 
and speed is down to 2-3gbps.
For this I will continue on this Add support for ASUS RT-AX89X AX6000 - #1089 by olci thread, as I see you are playing with fans there.
Out of curiocity, what is the difference between your config and default one except target for ax89x and disabling initramfs?
oh and I have one another problem I cannot install any package but this might be related to this https://github.com/openwrt/openwrt/issues/21981 I guess?