Just saw this and figured id chime in. If youre worried about weather sealing and performance on a 70-200mm, honestly, I wouldnt risk it with budget brands. Tbh I've always been the type to spend a little extra for peace of mind. If you want something that actually adds value to your kit, look at the Canon Mount Adapter EF-EOS R with Drop-In Variable ND Filter. It is pricey, but being able to use an ND filter behind the lens is a total game changer for video or bright daylight shots since it works with all your EF glass. If you dont need the filters, just grab the standard Canon Mount Adapter EF-EOS R. Its boring but it works every single time without any weird lag on that Sigma 35mm Art. Better to buy once and cry once than have your expensive lens wobble on a cheap mount.

Had a moment to think on this... honestly, the price gap between official gear and the Viltrox EF-R2 Mount Adapter is huge. Before I suggest the budget route, are you actually gonna use that extra control ring for your workflow? If you dont need the dial, you can save a ton with the Meike MK-EFTR-A Auto Focus Adapter. Since its basically just a pass-through for the pins, AF performance usually stays identical.

Honestly, congrats on the R6! Thats a solid upgrade. I went through the same dilemma when I switched over from my DSLR. Personally, I tend to be pretty cautious when it comes to adapters because I've heard some horror stories about third-party ones having fitment issues or the weather sealing not being quite up to par. I'm not 100% sure if the lag is a real thing across the board, but I think someone told me that certain off-brand adapters can struggle with older Sigma firmware, which might be a concern for your 35mm Art. IIRC, the basic Canon adapter is basically just a hollow tube with pass-through electronics, so theres no glass to mess with your image quality. I've always felt that if you're putting a heavy lens like a 70-200mm on there, you really want the most robust connection possible. I've seen some cheaper ones have a tiny bit of 'play' or wiggle when mounted, which would honestly drive me crazy and make me worried about the pins losing contact. If you can swing it, I'd probably just stick with the official Canon stuff. I'm not sure if the control ring version is worth the extra cash for everyone, but having that extra dial is kinda nice if you're used to having more physical controls. But yeah, for reliability and peace of mind, the official basic one is the way to go imo. It's just not worth risking a weird glitch in the middle of a shoot just to save a few bucks.