Do you need 3D Blu-Ray in 8k resolution to reach the point of diminishing returns when watching these videos with the 8kX?
Yes. Having done some stuff in Virtual Desktop with an 8kX unit, I know the angular resolution is closer to a virtual 8k screen than a virtual 4k screen. There are good reasons we use supersampling at higher resolution when rendering content to VR headsets, and this always applies in 3D space.
This is even more true if the virtual screen is very large (~180deg) and the footage is correspondingly suitable for the larger ‘IMAX’ displays.
You can one of the best ‘IMAX’ theaters out of the 8kX, albeit the headset is heavier, less comfortable, and more alignment sensitive than the polarized glasses used at theaters.
As the question is originally stated though, Do we need 3d blu-rays in 8k to fully enjoy Pimax 8kx?
No. Not unless all you do is watch movies. Seriously, there is more to life, especially in VR. Learn to fly little bird!