Oh man, I totally feel u on this. When I first got my R6 II, I was sooo worried my old flashes wouldnt work right with that new shoe design. I actually ended up trying a few setups because I didnt wanna drop a ton of cash right away... it was a realy stressful wedding season lol. Honestly, I found that you dont have to break the bank to get great TTL and HSS. I am still kinda learning the RF system, but here is what I recommend:

• Canon Speedlite EL-5
• This is the standard choice. It is super fast and works natively with the R6 II shoe, but it only works on the newer bodies.
• Godox V1-C Flash for Canon
• Tbh, this is my favorite. The round head is great for portraits and the battery life is insane for long receptions.
• Canon AD-E1 Multi-Function Shoe Adapter
• If you have older Canon Speedlite 600EX II-RT units, just get this adapter. It makes them feel secure. Basically I learned that while the Canon Speedlite EL-5 is cool, Godox is lowkey just as reliable for half the price. Just get the adapter if you use older gear so it doesnt feel wobbly! GL!

honestly, the Godox V1-C Flash for Canon is a total beast for the R6 II shoe cuz it balances power and weight perfectly! Just make sure you get the version with the updated metal foot to avoid connection issues.

• reliable TTL and HSS
• killer recycle times with Li-ion Its way more cost-effective than the pricey EL series and fits the mirrorless vibe perfectly. Love it!!

Late to the party but I've been wrestling with this exact setup for a few months. Like someone mentioned, those pins on the R6 II are super delicate and it really stressed me out when I first switched over. I tried using my old gear with the Canon AD-E1 Multi-Function Shoe Adapter but it felt kinda clunky and added a lot of height to a camera that is supposed to be compact. Are you planning on staying strictly on-camera for those weddings, or are you looking to build out a remote setup later? I ask because I ended up picking up the Canon Speedlite EL-10 and it's been a solid middle ground for me. It's way lighter than the EL-5 and sits better on the mirrorless body, though you do lose things like the weather sealing. I also tested the Godox V860III-C for Canon because the price is so tempting. In my experience, the Godox is a beast for raw power, but the TTL can be a tiny bit inconsistent on the new shoe compared to native gear. Canon just seems to talk to itself better, which saves time when things are moving fast. Tbh if you're just doing portraits the Godox is fine, but for weddings where you can't redo a shot, the native communication on something like the EL-10 or even the bigger EL-5 is worth the extra cash imo.

Similar situation here

• I went through this exact same thing last year when I upgraded. Honestly, I spent way too much time obsessing over whether my budget would survive the switch to the RF system. When I first got the R6 II, I was reallyyy worried about that multi-function shoe too... it looks so much more fragile with those tiny pins at the front, right?? I originally looked at the official high-end options from the manufacturer, but I just couldnt justify the cost when I needed two or three units for a full wedding setup. I ended up trying out some of the popular third-party Li-ion flashes instead. I think I heard somewhere that the connection can be finicky if you dont tighten the locking ring all the way, but so far, I havent had any misfires. The weight balance is the biggest thing for me. Mirrorless bodies are so much slimmer, so putting a huge top-heavy flash on there feels kinda awkward after a long day. I found that going with a third-party brand that uses those rechargeable lithium battery packs was a lifesaver. Not having to carry around a mountain of AA batteries is seriously amazing, and the recycle times are lowkey faster than my old gear. Not sure if this is 100% true for everyone, but I feel like the TTL accuracy on the newer third-party stuff has finally caught up. For the price of one native unit, I got a whole kit with a wireless trigger. It might not be as pro as the top-tier gear, but for my portrait work, it works well and I have no complaints!! TL;DR:
• Third-party units offer way better value for multi-light setups
• Li-ion batteries are a must for weight and speed
• Connection issues are rare if you get the updated metal versions
• TTL performance feels solid even on budget brands Hope that helps, the R6 II is a beast!! cheers.

Interested in this too

Just wanted to say thanks for everyone chiming in. Super helpful discussion.

Building on the earlier suggestion about Godox, you really need to be careful with the new electronic interface on that R6 II shoe. Unlike the old DSLR days, those 21 pins are exposed and extremely fragile. If you are doing weddings, reliability is literally everything, so I would suggest sticking to the Canon Speedlite EL-5. It was designed specifically for this new shoe architecture and you wont have to mess with adapters or worry about weird TTL lag. If the EL-5 feels like too much, the Godox V860III-C for Canon is a solid workhorse, but you might want to consider how much you trust a third-party foot on those tiny pins when you are moving fast. I have seen guys bend pins by forcing units in too quickly during a reception. Also, if you still have older EX flashes, do not just jam them on there. Make sure to use the Canon AD-E1 Multi-Function Shoe Adapter to keep the weather sealing intact and protect the mount from physical stress. It looks clunky but it saves your gear. TL;DR: Stick with the Canon Speedlite EL-5 for the best native performance, or at least grab the Canon AD-E1 Multi-Function Shoe Adapter if you plan on using anything else. Dont risk your pins to save a few bucks.