Regarding what #1 said about "I went through this exact same nightmare when..." - honestly same. It is so disappointing that Canon made the switch to mirrorless so clunky for those of us with older gear. I had issues with my 600EX-RT too and it basically ruined a couple of event shoots for me before I gave up on it. You dont actually have to spend EL-1 money to get reliable results tho. I know people are paranoid about the pins but as long as you are careful when mounting them, the third party stuff is fine. If you are on a budget but need that recycle speed:

• Godox V860III-C TTL Li-ion Flash for Canon
• Westcott FJ80 II 80Ws Speedlight for Canon tbh I actually prefer the Westcott interface anyway... the touchscreen is way easier than diving into menus during a reception. Just make sure you update the firmware first thing. That usually fixes any ETTL weirdness on the newer R bodies. You're gonna kill it at that Chicago wedding!

To add to the point above: > since I switched to mirrorless the communication feels... off. Like the ETTL is constantly overexposing Unfortunately, Canon really botched the transition to the multi-function shoe on the R6 Mark II. The issue you are having with the 600EX-RT is a documented synchronization lag caused by the digital handshake. Even if you use the Canon AD-E1 Multi-Function Shoe Adapter, I have had issues with high-speed sync (HSS) failing because the analog-to-digital translation just isnt as reliable as the old direct-pin contact. It is frustrating to see a decade of reliability disappear with a new body. If you are worried about the Godox shoe pins, you are not being paranoid. The early Godox V1-C Li-ion Round Head Flash units had a legacy foot that could potentially bridge the wrong pins on the R6 II. However, the newer Godox V1Pro-C Canon Speedlight has a redesigned shoe and improved thermal management. For your dance floor sets, the data shows it can handle nearly 100 consecutive full-power bursts without the thermal throttling that ruined the original V1. Honestly tho, I have been disappointed by the E-TTL consistency on the R6 II with most third-party gear lately. If you want to stay within that 500 budget and avoid the EL-5, the Westcott FJ80 II M 80Ws Universal Speedlight is a solid alternative. It seems to have fewer communication bugs with the Canon mirrorless protocol and the recycle time is a snappy 1.5 seconds. The color temperature stability is also rated at plus or minus 200K, which is crucial when you are shooting weddings in mixed lighting.

I went through this exact same nightmare when I first got my R6 II. My old flashes just werent playing nice and it was super stressful during fast paced events. I ended up biting the bullet on the Canon Speedlite EL-5 Multi-Function Shoe Flash and I have been incredibly satisfied with how it handles. No more overexposure bugs and it feels way more reliable than using an adapter or older tech. I shot a high energy reception recently and the recycle speed kept up with every single burst I took. It is honestly such a relief not worrying about it mid-shoot. Here are two quick tips to save your sanity:

• Stick with the EL-5 because the native multi-function connection is way more stable than third party options on the newer R bodies.
• Gently wipe the camera hot shoe pins with a dry microfiber cloth before the ceremony starts. Those tiny gold pins are sensitive to any oils or dust which usually causes those weird communication drops.