sooo, i feel u on the switch from the 5D IV—it's a massive upgrade! Basically, these adapters are electronic pass-throughs, but signal timing and internal tolerances are CRITICAL. If the data flow is off by even a fraction, third-party AF algorithms start hunting or the IS acts funky cuz it's not getting the exact handshake it expects. In my experience, here is the breakdown:
* Canon Mount Adapter EF-EOS R: The gold standard. I use it with my Sigma 50mm f/1.4 DG HSM Art and it's actually snappier than on my old DSLR. Zero communication drops.
* Viltrox EF-EOS R Mount Adapter: Great price, but I’ve seen random "Err 01" messages and IBIS jitters when paired with the Tamron SP 70-200mm f/2.8 Di VC USD G2. Honestly, dont sacrifice performance for price. Stick to the Canon one for a seamless experience. gl! 👍

Hmm, I've had a different experience. Over the years, I've found that 'simple' pass-throughs often fail with third-party signal timing. I'd actually go with the Viltrox EF-R2 Control Ring Mount Adapter. Unlike basic options, it has a USB port for firmware updates which literally saved my Sigma 35mm from constant hunting. Plus, that control ring is lowkey essential once you get used to it, right?

tbh I spent way too much time doing market research on this before I finally pulled the trigger. Basically, the whole 'handshake' between the camera and third-party lenses like your Tamron G2 is super sensitive to even tiny voltage drops. I started with a really cheap no-name one from a random site and it was a disaster—my Sigma 35mm would just freeze up mid-shoot. I eventually landed on the Commlite CM-EF-EOS R Electronic Auto-Focus Mount Adapter because I noticed it consistently getting decent reviews for having a tighter fit. From what I’ve gathered looking at the market landscape, most of these third-party brands are just reverse-engineering Canon’s protocols, but the build tolerances vary WILDLY between batches. If the adapter has any 'play' or wobble when you mount a heavy lens like that 70-200mm, the pins lose contact for a split second and the whole system crashes. idk if its just luck of the draw, but the Commlite has been SOLID for me. It feels way more cost-effective than the native one if youre on a budget, but you definitely have to be careful about the mount tightness. If it feels even slightly loose, just return it immediately cuz that’s where the AF hunting and IS glitches usually start.

Re: "tbh I spent way too much time doing..." - honestly it is so draining just trying to find gear that wont fail you in the middle of a gig. I went through the same thing and it is seriously a headache when the communication between the body and the glass just... stops. These third-party lenses like your Tamron are so finicky about the signal they get. It is just stressful and I hate that we have to worry about this stuff instead of just shooting. I would suggest being really careful with the off-brand options because the build quality on the internal pins is usually where they cut corners. If those pins dont make perfect contact every single time you are gonna get error codes or that annoying AF hunting. Quick tip from a DIY perspective: make sure to check the alignment of the mounting plate yourself before every big shoot. I have seen some slightly loose screws on the Metabones Canon EF to RF Mount T Adapter that caused tiny light leaks or connection drops. Just be careful out there, this transition is a mess tbh.

Respectfully, I'd consider Meike MK-EFTR-C Drop-in Filter Mount Adapter EF to EOS R over Viltrox. It’s cheaper and includes VND filters. Slightly heavier tho, but it’s a decent option for ur glass!

Came here to say the same thing lol. Great minds think alike I guess.