As u/michrech said, lag-free gaming is not possible. There will always be lag at some point. There are hundreds of posts in this subreddit about lag.
Your friend has the right idea, a dedicated WiFi6 AP wired to your router via a PoE switch. Place the AP in the same room that you will likely game wirelessly.
Do you already own the ASUS RT-AX86U Pro? If so, try it before you buy something else. 'Gaming' is mostly just marketing BS. In the consumer world 'Pro' doesn't mean much, either. The ASUS RT-AX86U looks like it will route 1 Gbps.
Whichever router you pick, disable its WiFi in favor of the 'real' AP.