@ecoin there is a new technique that @sebavan wrote a blog post about for real-time PBR filtering of light probes which may get you where you want to go. But that is going to be a bit expensive as it needs to be computed every frame so could be challenging if you target low end devices. It is also overkill if you don’t have dynamic objects in your scene so you will need to render once and keep that.
If you want to create a true HDR IBL in scene, we can’t do that as we don’t render HDR values into render target textures. And we don’t have a way to distort the image to make the cube seamless, so you would have to do that manually. In this thread, I go over the process of taking a panoramic image and adding the distortion to it before using it as a cube map. But unfortunately, if you are trying to render a prefiltered IBL, you won’t be able do to it from the scene due to the lack of HDR data in the scene.