f2.8 isn't exactly a fast lens. So, really, anything below that. I have an f1.8 prime on my camera that cost me, hold on to your seatbelts, a whopping $150.
And no, it's not the solution, it's A solution. A photographer/videographer would have many ways to deal with low light, including something as simple as moving your shot.
The price of the body has almost nothing to do with its low light capability. In fact, the 6D, with a newer sensor, actually does slightly better at high ISO.
Also, again, the video you showed us does not compare any noise at all, since it's physically impossible to see the noise with the heavy YouTube compression.
I know multiple people who have combined shot more weddings than you would in your lifetime. Setting up studio lights for video is a joke. "Here's all this space you rented, but I need you guys to sit in this tiny area the whole reception because that's the only place my camera can capture you without looking like an out of range TV station from the 70's".
"Crap loads", more like "I don't even know what these words mean". XAVC does not require a higher bitrate, and as I showed, the Canon shoots at a bitrate that's nearly 3 times as high. Bitrate will trump your ridiculous 8-bit/chroma crap every day of the week. Especially at the resolutions you're trying to record at. Bluray even uses 8-bit/4:2:0 and you think that it's still necessary? Yet ironically, even the lowest bitrate used is much higher than your camcorder can pump out. Odd how that works. But no, I'm sure you know better than several companies' worth of experience and testing.
Let's not forget the best part: You put so much emphasis on chroma and color, that you ignore the reality of capturing at higher levels. Doing so requires higher bitrate to produce the same level of compression quality. You are effectively compressing the video less, but since you are still using a low bitrate, the encoder has to make up for it by compressing the image itself more heavily. At the bitrates you're recording at, it's likely doing more damage than it's helping.
You're not shooting the next Sin City, there is literally no reason for you to need this.
The irony here is that 4:4:4 and 10-bit has nothing to do with those compression artifacts. Your lower bitrate does. Gee, funny how that works. Of course, the fact that you didn't even notice the heavy YouTube compression really demonstrates that you have no idea what you're looking at.
No, see, it won't happen to professionals, and you know why? It's because they keep backups. Because they're not so ignorant as to just walk into a shoot and hope that their equipment doesn't blow up, or their battery didn't charge, or that their camera suddenly won't turn on, or that their SD card suddenly corrupts. Multiple recordings or multiple bodies.