I've done much the same as the OP, having tried the Sony system twice with the A7R and then the A7R II, and ended up keeping my Canon gear both times. However, I could see the benefits of the Sony system (EVF, dynamic range, high ISO performance, eye AF, smaller form factor) and decided to give the A7R III a try. I was very pleased to find that Sony has fixed the biggest problems from the earlier cameras - crappy menu system, really crappy AF, and even crappier battery life. After using the camera for a few days, I bought the FE 100-400mm GM lens, and the A7R III + 100-400 GM combo actually outperformed my 5D IV + 100-400L IS II in my backyard focusing tests. To make a long story short, my Canon 5DsR and 5D IV are now gone, and a Sony A9 and a few more lenses are on their way to me this week.