Depends on what you're filming to be honest.
From a controlled environment point of view, forget upgrading the camera, focus on lighting. A 5D3 looked like garbage in a poorly lit scene when I was editing it. With good lighting, we pulled gorgeous footage from a T3i in a very dimly lit auditorium. From what I've read, the 5D4 is much better.
In the wild, look at actual video footage, and pay attention to which lenses are available. From personal experience, working with adapters is a bit of a hassle. Not a deal breaker, but I've heard stories of lenses getting stuck to the adapter. Also, don't lose sleep over lack of 4K or SNR charts. We had 1080p footage displayed on a theatre sized screen, and we received zero complaints over the quality of our footage.
The best thing to do is just go watch test footage, try to find some ungraded if you can. Once you've watched the footage, do what's kind to your budget. If you're happy with a 6D2, get a 6D2. There are many other aspects to video outside of resolution.