Hey Damian, thanks for reaching out!
I usually recommend sticking to the same brand and model cameras as much as possible for a multi-camera setup. That way you have image quality and colors that match as close as possible, so it's more consistent and less distracting when you switch between cameras.
I know the G21 has been discontinued, though. So you would have to get a used/refurbished G21 to match your current camera. That could be one option.
Another option you may consider is selling the G21 or giving it away and using your budget for 2 matching cameras. They might be cheaper cameras than the a6400 or BMPCC 4K, but they would be matching.
Now, if it's just a choice between the a6400 or BMPCC 4K then I'm going to recommend the BMPCC 4K almost every time. I don't recommend the a6400 at all for multi-cam or live stream situations because I've seen too many of them overheat and have power issues when left on for longer durations.