So, for your situation, I would suggest being realy careful with just the basic adapters. Honestly, I had some issues with the Canon Mount Adapter EF-EOS R when using my heavier glass. It's built fine for standard stuff, but with a 400mm prime, I noticed a tiny bit of play that was actually kinda terrifying during a shoot. Unfortunately, Canon doesnt make an official version with a tripod foot, which is sooo annoying for us pro users. I tried the Viltrox EF-EOS R Mount Adapter with Tripod Foot and while the foot helps support the weight, the AF was not as good as expected compared to native. If you stick with the Canon Control Ring Mount Adapter EF-EOS R, just make sure you're always supporting the lens and not the body. It's highkey a stressor on the mount... maybe look into a cage for extra security? gl!

Sooo I went through this last year when I made the jump to the R5. Quick reply while I have a sec! > Which EF to RF adapter is best for heavy professional lenses? Honestly, I was super worried about that 'play' everyone talks about with heavy glass. Since the official Canon ones mentioned earlier are kinda pricey, I looked for budget value to see if I could save some cash without my gear falling apart. I ended up testing a few third-party ones to see how they'd handle my 400mm. - Viltrox EF-R2 Mount Adapter: I found this one built like a tank. Pros: It's got a programmable ring and felt way cheaper than the Canon version. Cons: It's a bit heavier than I expected, maybe?
- Commlite CM-EF-EOS R Mount Adapter: This was my super budget pick. Pros: Dirt cheap and came with a detachable foot. Cons: The finish isn't as nice as the Viltrox, I guess. I'm actually *really* satisfied with how the Viltrox held up. No wobble even when I'm swinging the 400mm around. It basically saved me a hundred bucks and feels just as sturdy as the pro gear it’s holding up! peace 👍

I am literally having this exact same issue with my 70-200 right now and it is so disappointing. You'd think Canon would have a better solution for heavy pro glass but even my official ones have developed a tiny bit of play over time. It makes me nervous every time I swing the camera around. Since I gave up on finding a perfect adapter, I basically had to rig up my own support system to keep things from flexing:

• I switched to a SmallRig 2982B Camera Cage for Canon EOS R5 which helps distribute the stress away from the mount.
• I added a SmallRig 2152 Universal Lens Support that attaches to the cage. This is the only thing that stopped the 400mm from wobbling.
• For the adapter itself, the Viltrox EF-R2 Mount Adapter actually fits tighter on my copy than the OEM one did, though quality control can be hit or miss. Its honestly not as good as expected for the price we pay. If you're shooting daily, the cage and support combo is the only thing that actually feels solid enough to trust.

In my experience, you really gotta be careful with those heavy setups. I lowkey had a nightmare experience where I felt the mount literally flexing under the weight of my 400mm prime... it was terrifying. AF kept dropping out too, which was honestly so frustrating during a pro gig. Basically, the basic adapter is fine for light stuff, but for daily pro work, it just doesnt feel beefy enough for that kinda glass. Here's what I recommend:
1. Definitely go with the official Canon version that has the built-in tripod foot. It adds that extra bit of rigidity you need for the primes.
2. Ngl, even with the good one, I still check the screws on the mount regularly cuz vibration is a thing.
3. If you're using the 70-200, always support the lens, never just the body. Seriously, dont risk your glass on a flimsy connection. I learned that lesson the hard way and it was realy a letdown. gl!

Can vouch for this

Honestly, I think people worry too much about the specific brand and not enough about how to maintain the gear themselves. I've heard from a few folks that even the high-end stuff starts to get some play if your using a 400mm prime daily. Not sure but I've been told you can actually tighten the internal flange screws yourself if you've got a steady hand and the right precision tools. If you're really worried about that flexing, you might wanna look into some DIY support options instead of just relying on the mount. I've seen some pros use:

• Long lens support brackets that bridge the body and the lens collar
• Small dabs of low-strength thread locker on the mounting screws to keep things from backing out
• Regular self-checks on the mount tension before a big gig Sending it in for a professional service is always an option if you don't feel comfortable messing with it, but for a daily pro, learning to do those minor adjustments yourself can save a ton of downtime. IIRC there's some good tutorials online about how to check those tolerances. Basically, no connection is gonna be 100% rigid forever with that kind of weight, so being ready to do a bit of self-service is probably the way to go.

Jumping in because I have been using this setup for three years now and honestly? I love it! When I first got my R5, I was freaking out about my 400mm prime just like you. I actually wasted so much money trying every fancy version out there thinking price meant stability.

• The super expensive one I bought actually developed way more wiggle than the cheap backup I got on a whim after just a few months!
• I learned that how you carry the rig matters way more than the brand of the metal bits. I always carry by the lens handle now and it has saved my gear so much stress.
• My biggest takeaway was that tolerances vary so much... my current setup cost me peanuts and it is still rock solid after hundreds of shoots! It has been a total game changer for me! Just goes to show you dont always need to spend a fortune to get pro results.