I don't generally use ProRes from 8 bit source and projects, it seems to introduce some pesky shadow noise in sensitive scenes. The noise is not evident in ProRes renders from 10 bit source.
Among 8 bit intermediates I have tried are MagicYUV, MPEG-2, XAVC-I, and Sony YUV, each with their strengths and weaknesses.
What are you using? What do you consider the best tradeoff between size, quality, speed?
When I used intermediate video formats in the past, I mostly used MagicYUV. Lately, MagicYUV has had some issues that put me off somewhat.
The projects that I've been doing for a long time have not required that I use intermediates. So, I haven't.
As a solution to decoding and/or timeline preview problems that others have had, I've recommended using Shutter Encoder to transcode to Apple Pro Res. Vegas Pro seems to work well with it. Doing so forces the user into 10 bit territory as well as using a huge amount of disk space. Sometimes this can be worth it to get something out the door.
I've not analyzed "pesky shadow noise in sensitive scenes" but, I'm willing to learn.
Disclaimers
The Question may be: What drives you to use an intermediate???