Run vbr without any overrides... try 30P if it freezes in 24

The algos are all in the encoder part of the digic, I'd have to find where to manually set current QP, I don't know if you can just do it on the fly... that location is "init" or start qp and is probably only read once.
CBR config is for fixed QP/override like you're doing. If you look in the IVA params thread on 5d3 some "max" rates were found before it starts overloading the endoder/writing. That point for CABAC/CAVLC is different and ALLI/IPB/IBB too.
The way this works is QP is quality, so QP 1 can be 30mb/s to 400mb/s depending on your scene. Running fixed QP with VBR gives you the extra profile stuff and CABAC but when you look at the files, every frame will be at that QP you set. (Why I kept saying don't set initqp with configs) When the frame becomes like 2MB and the buffer gets hit with 24 of them, if the writing can't keep up the encoder drops frames. It can't really do anything else, it can't lower QP, you told it not to.
The RC will go up to 100MB/s and over if your scene is complex enough. Some scenes are just 60mb/s, they are static with not much going on. When you look in the file, QP is 1 already.
Tested it before flush rates and evened it out. The old one had a limited range of QPs and then when scenes got big and it ran out it did something similar (truncated frames). You can tweak the min/max to give you higher rates but I kept it 1-30 so it wouldn't run out, esp with upped flush rate.
BTW it is possible to get a clean file at ~300-400Mb/s fixed QP1, just its very hit or miss.