You have to understand the UHH Canon fanboys will be quick to strike here if you even dare to suggest leaving Canon for Sony, or any other brand, but certainly not to Sony OMG, LOL

As a former longtime Canon and Nikon user that moved over to Sony, I have a different perspective than others. I suggest you look at the excellent fullframe 33mp Sony A7IV; or the even smaller and lighter fullframe 33mp Sony A7CII or fullframe 61mp Sony A7CR cameras. Either will give you great performance with the superb Sony 200-600mm lens.

As far as mirrorless-designed lenses go, the Sony E-mount is the most widely produced and used mirrorless lens mount in the world with over 200 + (and growing) choices from Sony and excellent third-party lenses from Sigma, Tamron, Samyang/Rokinon, Zeiss, Tokina, TTArtisan, Viltrox, Meike and others. No other mirrorless camera brand has as many mirrorless-designed lenses as there are for Sony E-mount.

And these E-mount lenses encompass many compact and lightweight, yet still excellent quality and affordable mirrorless-designed lens choices from Sony and all the third-party makers. Third-party makers today leaked another 13 new E-mount lenses coming before the end of the year, as well as new ones coming from Sony.

https://en.wikipedia.org/wiki/List_of_Sony_E-mount_lenses

https://en.wikipedia.org/wiki/List_of_third-party_Sony_E-mount_lenses


If you prefer higher megapixel resolution and still want to go for Sony, by all means the 61mp A7RV or 61mp A7CR are superb and less costly than the higher frame-rate 50mp A1

1) Here 50mp Sony A1, Sony 200-600mm lens. A Snowy Egret takes off from its watery perch on the island of Sint Maarten/Sint Martin. 319mm, ISO 1600, f6.3, 1/4000 sec. handheld. Bird Eye-AF used.

2) Here a shot with my 61mp Sony A7RIV, the same 61mp sensor in the new compact Sony A7CR, or Sony A7RV. The Golden Gate Bridge in San Francisco, California, USA. Sigma Art 24-70mm f2.8 DG DN lens. 24mm , ISO 200, f8, 1/80 sec., handheld, late afternoon light.

Click in download to see better image quality.

Sony A7CII and A7CR Camera Review: Mark Galer
https://www.youtube.com/watch?v=5iq3Mn85LNk

Sony A7RV:
https://electronics.sony.com/imaging/interchangeable-lens-cameras/full-frame/p/ilce7rm5-b

Full disclosure: After 40+ years shooting pro Nikon, Canon, Leica, and winning National and Worldwide Photo Awards with them all, I moved over to Sony mirrorless and have never regretted it for all my professional and personal work shooting all subjects, all around the world. I have owned Sony A6500, A7RII, A7RIII, A7III, A9, A7RIV, and I currently own A7RV, A1, A7SIII. I currently have 13 E-mount lenses covering 10mm to 600mm from Sony, Sigma, Tamron, Viltrox.

And interestingly, though not perhaps for what you shoot, top Worldwide News Services have moved exclusively to Sony and Nikon mirrorless for all the staff photographers and staff videographers, worldwide. None of the top Worldwide News Services chose to move to Canon mirrorless for their staffers, none.

Associated Press, all Gannett media including USA, Canadian Press, UK's top News Service all chose to move exclusively to Sony mirrorless for all their staffers worldwide, after years of testing Sony gear in the field around the world. AFP (Agence France Presse) helped develop and test the Nikon Z9, and they use Nikon Z9 for their staffers worldwide.

https://alphauniverse.com/stories/why-the-associated-press-just-switched-to-sony/

https://petapixel.com/2021/11/17/sony-is-now-the-exclusive-camera-provider-for-gannett-and-usa-today/

https://www.dpreview.com/news/4545693607/the-uk-largest-news-agency-partners-with-sony

https://petapixel.com/2022/01/31/canadas-largest-news-organization-moves-exclusively-to-sony-cameras/#:~:text=Canada's%20Largest%20News%20Agency%20Moves%20Exclusively%20to%20Sony%20Cameras,-Jan%2031%2C%202022&text=The%20Canadian%20Press%2C%20the%20largest,provider%20for%20the%20media%20company.

https://petapixel.com/2022/06/09/how-pro-photographers-helped-make-the-z9-from-prototype-to-flagship/

Cheers and best to you, whatever you decide.

It may help if we knew the model of your Canon body.
Maybe it is too early in the morning but I don't understand the "Need" for "Back Button Focus" when both "Eye Focus" and "Focus Tracking" are available.
Both "Eye Focus" and "Focus Tracking" when combined do a much better job than I could ever do using MF on my Canon R5.
This conundrum has puzzled me for some time, and I truly hope to learn something today.
Best Wishes,
JimmyT Sends

Edit: The YouTube video previously recommended by Tracy B, "Canon R5/R6 Fast Eye Autofocus Switching | Ask David Bergman", makes a lot of sense to me.