Magic Lantern or native specs in 2020
You'll see an improvement with the 5Diii over the 77D without installing Magic Lantern, because the former has a full frame sensor, and the latter only an aps-c sensor.  Then, you're free to experiment with ML, use it if you like it, return to the normal firmware if you don't.  The major limitation that ML overcomes (in my opinion) is the maximum video duration.  Yes, it adds other bells and whistles, but it won't alter the fundamental physics of the camera, which is what you're getting by upgrading to a larger sensor.