After looking at this from a mechanical stand point, this style of frame is just wrong.
If you look across the frame diagonally you can see that lines drawn from motor center to motor center line lines up intersect at roughly the physical center of the frame.
If you look from a motor center to the arm pivot point (where all yaw torques are applied) the actual motor torques lines meet approximately 2"/5cm aft and approximately 3"/7.5cm forward of the physical center of the frame.
I do not believe that any amount of tuning will mitigate this inherent design flaw and I am looking for a new frame…
I tend to agree. I had a frame with similar geometry, kind of large Dead Cat. I was never happy with the performance of that craft. I ended up just using Quad X but I recall Yaw authority was poor no matter what I tried and overall it was a dog. I ended up scrapping it and put the parts on the shelf for other builds so it wasn’t directly replaced.