Just installed Fiber internet. Can I keep WIFI on both BGW320-500 and connected ASUS RT-AX88U

1. Just installed Fiber internet. Can I keep WIFI on for both BGW320-500 and ASUS RT-AX88U router. BGW router locates in the garage and the ASUS is at the far end of the house.

Yes but devices may not just roam between the Wi-Fi signals. Are you planning to use the same SSID an password on both systems?

2.  Is passthrough a must step for my ASUS RT-AU88U connected to BGW? 

No you have the option to setup your ASUS in Repeater mode or Access Point mode if there is an (Ethernet connection) from your BGWxxx to your ASUS.

IP/Passthrough mode will allow you to use more of your ASUS Router functions.

Dave

1. I plan to switch from Xfinity DSL to AT&T Fiber. Currently, my ASUS router is connected to the Xfinity cable modem via an Ethernet cable, with each device having its own SSID and password. When I switch to AT&T Fiber, I intend to use the same SSID and password for both the AT&T modem and the ASUS router. I will simply connect the ASUS router to the AT&T modem's Ethernet port.

2.  Actually I just want the quickest and simple steps to finish the setup and enjoy the faster network.

Be aware that if you have two routers on a single connection you run the risk of a double NAT, which can cause all kinds of issues. As mentioned, either use the Asus in Access Point Mode and leave the AT&T gateway alone or setup the AT&T gateway for IP Passthrough and allow the Asus to handle all of the routing  and WiFi duties. You'd probably get more reliable WiFi that way. Just see which way works best for you and your environment.