I went from a 5D MKii to an R5. Admittedly not the type of jump you are making, and the AF on the MK ii is worse than the mk iii, but it was literal night and day. I went from getting 2 out of ever 10 photos of my dog (who is 99% black) to getting 9 out of every 10 photos with nailed focus. The AF is absolutely amazing, and yes you can use all your EF lenses with an adapter and there is no optical detriment as there is no glass or optics at all in the adapter, it is just a spacer. The Eye AF is brilliant and I now use dual back button AF so I can hold one button down and it will focus on to the eye of the subject or I can hold a different button and it will use the AF point. Very flexible and feels like cheating when you first start using it. If I were you, take a couple of your favourite lenses and go to a second hand camera shop and try out the R8, R, and the R6. That will give you a good selection. Any of the full frame R cameras will be an upgrade in the AF and dynamic range department. The EOS R uses the same sensor as the 5D mk IV for example, and that is the oldest of all the full frame mirrorless R range. Regarding the EVF and refresh rate, the R can go up to 60hz which is ok but for many people feels like a downgrade vs optical view finders. The r8 and the R6 both go to 120hz which is what I use on my R5. It means your battery life takes a bit of a hit but it is so much better and not laggy at all. Great for taking pictures of fast action. Then you can fully appreciate being able to instantly see exposure through the EVF and not be distracted by the poor response times. Make sure if you try the cameras out, to set the EVF to 120hz in the settings. I am sure someone in the shop would be able to assist you with that. The cameras will default to 60hz. Answer from Qazax1337 on reddit.com