Hey, I am just reading in as i am researching some of the same challenges. Quite happy to see that you are still on the project after such a long time. One of my main goals was to be able to get CVBS on my Blackmagic Video Assist (field monitor with integrated ProRes (Broadcast Quality) Recorder, and I happen to have found a converter that might be low signal friendly: https://www.aliexpress.com/item/32897673378.html?spm=a2g0s.90423184.108.40.206424c4dYjTfU1
First test were quite good, as it seems to have some sort of signal sync generator, but I have yet to test this recorder in the field.
Just so that you know, this is what a recording on ProRes looks like with CVBS footage: https://youtu.be/ID-fd5XVzuA
It just so happend that this converter can also see AHD, so I purchased a bunch of AHD camera's, that got sidelined to another project and are currently installed in a birds nest, but I'll get some more and share on the researching part.