Quick reply while I have a sec! Story time: I went through this exact same thing last year after finally grabbing an R6. Like, I spent my whole budget on the body and had almost nothing left for accessories, but I still had all this 5D glass... tbh I was terrified of my heavy 70-200 just falling off. I tried to go the ultra-budget route because I'm always looking for a deal, but man, it was a bit of a rollercoaster... I guess I'm still learning the ropes with the RF system, but just sharing my experience with the non-Canon stuff I've messed around with: * Commlite CM-EF-EOS R Mount Adapter: This was my first one cuz it was so cheap. Honestly? It worked okay for static shots, but like you mentioned, there was definitely some tiny wiggle when I was moving fast. I always felt like I had to check if it was still locked in, which is basically a nightmare for event work. * Meike MK-EFTR-C Drop-in Filter Mount Adapter: I moved to this one later cuz I wanted the drop-in filter feature. The build felt way more solid than the Commlite, but I still had one weird moment where my AF just... stopped?? I had to unmount and remount it to get it talking again. Anyway... I guess what I'm saying is that while these "value" options are cool for the price, that "play" you're worried about is real. I know others mentioned the first-party stuff already, and I've started to think they're right about the reliability. I love my EF glass too much to risk a connection drop during a big moment, so I've become super cautious. It's a tough call when you're trying to save cash, but the connection is literally the most important part of the rig. GL with the new camera though, you're gonna love it! Peace

Respectfully, I'd consider another option before writing off the higher-end first-party mounts. I've had a bit of a different experience than people who say all adapters feel the same. Honestly, I tried to save money with a Viltrox EF-EOS R Mount Adapter, but unfortunately, the machining tolerances just weren't tight enough. During a wedding, my Canon EF 24-70mm f/2.8L II USM would literally lose connection if I put any lateral pressure on the lens barrel while moving. It was a nightmare. Basically, if ur looking for technical reliability, the Canon Mount Adapter EF-EOS R and the Canon Control Ring Mount Adapter EF-EOS R are the only ones I’d trust for pro work. The stainless steel mounting flanges on these are machined to much higher standards than the 3rd party stuff. I've found the connection on the Canon Control Ring Mount Adapter EF-EOS R to be just as secure as the standard one—no extra 'play' at all, which was a huge relief since I was worried the ring might add bulk or instability. I mean, I know brands like Meike MK-EFTR-A EF to RF Adapter are popular because they're cheap, but the weather-sealing gaskets are noticeably thinner. If ur shooting events and things get a bit messy, that’s a big risk for ur Canon EOS R6 Mark II sensor. Stick with the first-party Canon mounts; the electronic pins are better aligned and the physical lock is SO much more confident. gl! 👍

Basically, if you look at the market teardowns, the big difference between the tiers is the mount material and the environmental sealing. Most third-party manufacturers skip the rubber weather gasket at the flange, which is a huge risk if you're shooting events outdoors where dust or a stray splash can happen. * Metabones Canon EF to RF Mount T Smart Adapter: This is a solid professional alternative because they use chrome-plated brass mounts on both sides, which is way more durable for heavy L-series glass than the aluminum used by cheaper brands.
* Commlite EF-EOS R Mount Adapter: Often cited as a decent budget pick, but the machining tolerances can be hit-or-miss, and they definitely lack the internal flocking needed to prevent light reflections. If you're pushing heavy 2.8 zooms, you definately want that brass-to-brass contact to avoid the mount wearing down over time. It's pretty much a "buy once, cry once" situation when you're dealing with a pro-level body like the R6II... anyway, just make sure whatever you get has a tight gasket.

I would suggest avoiding 3rd party mounts:
- Wierd play issues
- Connection drops
Be careful, stick with Canon... honestly its way safer for ur L-glass during busy pro shoots.

I had the exact same worry when I moved to the R6 Mark II. My heavy EF 70-200mm f/2.8L II felt like it was gonna snap something if I moved too fast. Tbh I was checking the mount for gaps every five minutes during my first few shoots because that play everyone talks about is a total head trip. Im actually really satisfied with how I solved it though. Instead of the pricey Canon ones, I grabbed the Commlite CM-EF-EOS R Mount Adapter. To make it pro-ready, I did a little DIY tune-up. I noticed that most connection issues come from the internal screws on the mount plate loosening over time from vibrations. I used a precision driver to make sure they were seated perfectly. If you still feel a tiny wiggle, a tiny piece of 3M Kapton Polyimide Film Tape on the mount flange creates a perfect, tight seal that stops any rotation. It works so well and gives that rock-solid feel you get with native RF glass. No electronic drops at all even during long handheld days.

I totally agree that the physical connection is the scariest part when switching systems, especially since your L glass is so heavy. Tbh I am still pretty new to the RF world but I have been looking into some DIY ways to keep everything tight without needing a pro service every time. Even if you get a solid one, I think it is worth knowing how to maintain it yourself so you dont have to panic mid-shoot, you know?

• Get a set of precision screwdrivers to gently check the screws on the adapter rings if they ever start feeling loose after a lot of handheld work
• Keep a specialized electronic contact cleaner or just a tiny bit of isopropyl alcohol on a cloth to wipe the gold pins whenever you start seeing communication errors Basically, if youre worried about the connection dropping, just keeping those pins clean helps a ton with the electronic hiccups. Idk if the professional shops would recommend doing it yourself, but it worked for me when my Meike MK-EFTR-A Auto-Focus Mount Adapter felt a bit glitchy at first. Does anyone else here do there own maintenance or is that too risky for the expensive L-series glass?