1. The Size: What happens when you take a professional lens like a 70-200 2.8 and slap it on a small frame like the a7r III? Its not good. Your hand will just fit between the grip and the lens if you'er a small guy like myself. The Canon 5D Mark IV fits very nice in the hand with plenty of room around the grip.
The button on the camera are also small and close together. Yes, you will get use to that but don't be surprised by all the fat fingering you new Sony will get!
2. The Menu: Ready for Spanish 101?? The Sony a7r III has 6 tabs and 37 total pages on in the menu system. Most of those option are going to be all new for you Canon lovers. Also, the menu is not touch enabled on the a7r III unlike the Canon 5D Mark IV.
3. Perception: This is one thing I hear nothing about. What will my clients say when they see me walk in to offer a professional service with a camera body that is smaller than a Canon Rebel?? Perception is the only reality. That is why I love the Canon 5D Mark IV. Its the perfect size!