I have two ASUS RT-AX92U in use. The AC86U described by michmouse doesn't really make sense in combination with the AX88U, because otherwise you won't benefit from the fast WLAN standard (WLAN 6 = AX, WLAN 5= AC).
In general, you can use any router for expansion, but you should make sure that it is
a) Ai-Mesh compatible
b) has "AX" in the article description

In principle, you can take any router and use it as an access point. In your case, however, I would consider using two Asus routers that support AiMesh. This simplifies configuration and all routers occupy the same WLAN channel, which is definitely an advantage in a densely populated area.

Hello, I am also using the ASUS RT-AC86U Gaming. Bandwidth and throughput ok.