Congrats on the upgrade! I moved from the 5D too, and honestly, i wouldnt look past the official Canon gear. If you care about weather sealing and tracking speed, the Canon Mount Adapter EF-EOS R is the way to go. Its basically just a hollow tube with pins, so there is zero lag or quality loss. Personally, i think the Canon Control Ring Mount Adapter EF-EOS R is worth the extra cash. Having that ring to change ISO or aperture on the fly makes the R6 Mark II feel much more intuitive. I tried the Viltrox EF-R2 Mount Adapter on a shoot once and it felt a bit loose... not something i would trust with a heavy lens like your 70-200mm. Your Sigma Art will work perfectly with the Canon adapters though, tracking is actually better on mirrorless than it ever was on my DSLR.

Just catching up on this thread. If you want that control ring functionality without the Canon price tag, definitely look into the Viltrox EF-R2 Control Ring Mount Adapter. I have used it and it feels really solid, though the weather sealing isnt quite as beefy as the official gear. Another decent budget pick is the Commlite EF-EOS R AF Mount Adapter. It is super simple and works fine, but lacks any extra features. I usually check out Dustin Abbotts reviews or The-Digital-Picture for deep dives on how these handle AF tracking speed. One practical tip: if you go third-party, keep the USB cable handy. They often need firmware updates to stay compatible with newer R series bodies. Honestly, for your big zoom, the Viltrox is a great middle ground for cost vs performance.

Just saw this and wanted to chime in on the technical side. I went through a similar phase trying to save some cash after dropping a fortune on the body. Tbh, these adapters are really just spacers with pins, so you arent losing data between the lens and the R6II. I ended up picking up the Viltrox EF-EOS R Mount Adapter and found the AF tracking with my Sigma glass was basically identical to the native feel. If you want to be smart with your budget, look into the Meike MK-EFTR-C Drop-in Filter Mount Adapter with Variable ND Filter. It lets you use one ND filter for all your EF lenses, which saved me a ton on individual filters for my 35mm and 70-200mm. The build quality is solid, tho the weather sealing might be a hair less robust than the Canon version... but for the price difference, it is really hard to complain.

Been using this for years, no complaints