Coming back to this, it seems like the consensus is basically choosing between the rock-solid Canon build or the utility of those drop-in filters mentioned earlier. To add some extra technical perspective, you should really look at the internal finish of these things. If you are using vintage glass, internal reflections are your biggest enemy because those old lenses aren't always multi-coated to handle digital sensor bounce-back.

• Viltrox EF-R2 Control Ring Mount Adapter: This is a fantastic middle ground. It gives you the programmable control ring for way less than the Canon version. I havent noticed any significant light leaks or weird internal flaring during my tests.
• Meike MK-EFTR-A Auto Focus Mount Adapter: This is my budget pick. Unlike the basic Commlite, this one actually has a rubber gasket on the mount side to help with weather sealing and stability. Basically, if you are stacking an M42 adapter on top of an EF-RF one, you want the tightest tolerances possible to avoid sensor plane tilt. The Viltrox is definitely the best value for money here if you want that extra functionality.

I've been shooting Canon for years and honestly, the Canon Mount Adapter EF-EOS R is the way to go. I know it's pricier than the basic metal tubes, but the build quality is night and day. Cheap ones like the Commlite CM-EF-EOS R can sometimes have a bit of wiggle which is super annoying when you're trying to nail manual focus on vintage glass. Since you're using M42 glass, you're already stacking adapters, so you want that base mount to be rock solid. I tried a budget Viltrox EF-EOS R Mount Adapter once and had some internal reflection issues in harsh light. If you can afford it, the Canon Control Ring Mount Adapter EF-EOS R is actually pretty cool because you can map that ring to ISO or focus peaking. Tbh, just stick with the official Canon standard one... it fits perfectly and wont let you down.

Honestly, if you are diving deep into vintage glass, you should definitely look into the drop-in filter options. Its a total game changer for lenses that have weird front diameters or those where the front element rotates while focusing. I spent way too much time hunting for obscure step-up rings before I grabbed the Canon Drop-In Filter Mount Adapter EF-EOS R with Variable ND Filter. It lets you use a single ND filter for all your old glass regardless of the lens size, which is huge for video or shooting wide open in bright daylight. If the Canon version is too pricey, and it definitely is a bit of a sting, I have had surprisingly good luck with the Meike MK-EFTR-C Drop-in Filter Mount Adapter EF to EOSR. Its built much better than the bottom-barrel brands and the filters actually slide in smoothly without those annoying light leaks you see on the super cheap stuff. One thing to keep in mind with your M42 glass is that you will be stacking a simple Fotodiox M42 to Canon EF Lens Mount Adapter onto the EF-RF unit. This adds another failure point for wiggle, so getting a high-quality EF-RF base is crucial. Definitely check out the Phillip Reeve website if you havent yet; they have some of the best technical guides on adapting manual glass to mirrorless. Also, Dustin Abbots site is a great resource for seeing how these older designs resolve on high-megapixel sensors like the R6.

• honestly ive been dealing with this exact same headache for about a month now and still havent found a clear answer tho - i keep looking at my shelf of old glass and then looking at the official prices but i just end up closing the tab because im so paranoid about third party reliability...
• it is just so frustrating being stuck in this limbo where i want that vintage look but i am too scared to trust a cheap tube of metal with my camera