Best way of Using OpenWRT RPi router and 2 downstream routers

I'm wanting the best way to setup my network using a RPi 4B, configured with OpenWRT, as the router connecting to my ISP.

I have 2 routers that I want to use to segregate my network. The first of the routers is the Asus AX92U with stock firmware. The second router is a TP-Link Archer A6 V3, which will run OpenWRT (once I get the router).

The Asus AX92U is part of a Mesh pair that I want to use the main LAN for the Smart TVs and other streaming devices. I am able to create guest networks and will use these for IOT devices.

The TP-Link Archer A6 I want to use strictly for the Computers, phones and tabs.

I do have a managed Netgear GS108E switch to use if needed.

I want to only have Single NAT as we play online games.

I'm honestly unsure how to configure this setup to make the network be how it should. I know I can configure guest networks and also configure the OpenWRT router with VLANS to further segment the network. I'm just unsure of how I need the hardware configured to make the thing work right.

I guess the Asus needs configured as an AP with Mesh. But I'm unsure of how the OpenWRT (RPI and Archer A6) needs configured to make it work.

Any help and insights would be appreciated