Jumping in here... im curious, do you really need that control ring functionality or are you just after the most stable AF? If you're trying to save some cash, the Viltrox EF-R2 Control Ring Mount Adapter is way cheaper than the Canon version. I've also seen the Commlite CM-EF-EOS R Mount Adapter for even less. What's your actual budget looking like for the adapter?

Honestly, i’m gonna have to disagree slightly with the "cheaper is fine" approach for professional work. If you’re pushing high-performance glass like the Canon EF 70-200mm f/2.8L IS III USM on an R6, signal latency is your enemy. If you want the ultimate setup, the Canon Drop-In Filter Mount Adapter EF-EOS R with Variable ND Filter is actually the pro choice.

• Pros: Allows wide-open f/2.8 shooting in high noon sun, superior weather sealing, no communication lag.
• Cons: Very expensive compared to standard tubes. Alternatively, the Meike MK-EFTR-C Drop-in Filter Mount Adapter EF to EOS R is a great middle ground.
• Pros: Much cheaper than Canon, includes the VND functionality.
• Cons: The seals aren't quite as tight and the dial feel is a bit grittier. Tbh, for eye-detection, you want the tightest mechanical tolerances possible. Any wiggle in the mount can mess with those micro-adjustments the AF system makes... just something to consider before going too budget.

Like someone mentioned, staying within the brand ecosystem is usually the gold standard, but i have been super happy with the Meike MK-EFTR-A Auto-Focus Mount Adapter. honestly i was a bit nervous about third-party communication at first but it works well on my R6. i use it with my 85mm prime all the time and the eye-tracking is snappy as heck. compared to the viltrox or commlite options, the build quality on the meike feels way more robust for daily use. i also messed around with the Meike MK-EFTR-C Drop-in Filter Mount Adapter which is a killer value if you want those filters later on without paying the crazy canon price. for just straight up reliable AF though, the standard meike has been solid for me... basically feels native and i havent had a single connection drop yet.

Been using this for years, no complaints

Honestly, just stick with the Canon Mount Adapter EF-EOS R if you want that snappy eye-tracking. I have used it on my R6 for ages with the 70-200 and it basically feels like a native lens. Its the most cost-effective choice compared to the Canon Control Ring Mount Adapter EF-EOS R while giving you identical AF performance. Third-party ones like Viltrox just arent as reliable for professional tracking tho.