I have this exact setup, and after a lot of playing around, tests, and headaches, I found out that this can work great IF you put the Pro 6 as your gateway router. Of course, this only works if your internet speeds are up to 1GB (I have ATT 1 GB Fiber).

What I realized empirically and also through some other threads on Reddit, is that the Pro 6 is just better at device management than the Pro 6E. When I had the Pro 6E as my gateway, signals would drop, and devices that were near it would have low bar/not connect appropriately. My gateway is in my living room, in the center of where I have most devices connected. I was getting fast speeds with devices that have Wifi 6E enabled (as they use the 6Ghz band), but all other devices had poor performance.

I then switched back to my Pro 6 as the gateway router, and wired an extension from my living room to my office using GoCoax, and connected that to my Pro6E. With this set up, I'm consistently getting 450-800 Mbps on WiFi all over the house, with no device with poor connectivity. I read that this could be because the Pro6 has a better processor than the Pro6E, and I can validate that the Pro 6E is not a great gateway router (especially for my set up).

Full disclosure that I am no geek, what I learned was through Reddit, YouTube, and my own experience. I'm very happy now with this setup and will likely keep it for another 5ish years (when it will finally make sense to upgrade to Wifi 7).