Seconding the Godox recommendation above! I used to be a Canon purist, but honestly, third-party tech caught up. On my 5D IV, I switched for the battery life and better weight distribution. • Godox V860III-C TTL Li-ion Flash for Canon - It’s a workhorse with way faster recycle times than AAs.
• The AF-assist is actually super reliable on the latest firmware. Lesson learned: Li-ion batteries are ESSENTIAL for 8-hour gigs. You'll save ur wrist too!

Sooo I went through this last year! I compared two main setups for my 5D IV: - Canon Speedlite 600EX II-RT: Literally indestructible and the AF-assist is perfect.
- Godox V1-C Flash for Canon: Way faster recycle times with that Li-ion battery. Honestly, the Godox is basically a lifesaver for long weddings cuz it’s lighter and easier to swap batteries, even if the AF beam is kinda hit-or-miss sometimes... good luck!!

I've been shooting with the 5D series since the Mark II days and honestly, if you want reliability without the Canon price tag, check out the Nissin MG80 Pro Flash for Canon. It is a beast and handles heat way better than the Godox units. Another one that is surprisingly good for the weight is the Nissin i60A Flash for Canon. It is tiny but packs a punch, which helps with that wrist fatigue you mentioned. Speaking of fatigue, I was actually looking at those new mirrorless bodies the other day because my shoulder just can't take the 5D weight anymore after a full day. I spent like three hours reading reviews on lightweight straps last night instead of actually editing my last gallery... it is a total rabbit hole once you start worrying about gear weight and ergonomics. I even started looking into those hip-holster things but they look so dorky. Anyway, the Nissin stuff is super underrated for pro work and should fit your budget nicely.

tbh I just saw this and I remember being super stressed about the same thing when I started doing weddings. If reliability is your main thing, you really should just go to YouTube and search for canon vs godox flash reliability for weddings or something similar. There are a bunch of videos where people show exactly how the AF assist works in pitch black rooms and how the colors compare between the brands. I found a really good one a while back that basically broke down the pros and cons of staying native vs going third party for the 5D series. Also, definitely check out the gear review threads on Reddit, those guys are usually pretty brutal about what gear actually fails during a long day. Its way better to see it in action in those comparison videos than just taking our word for it here, especially since you dont want to risk a malfunction during a big moment like the cake cutting.

Yeah, I totally hear what everyone’s saying about those other brands, and they definitely have their fans, but I’m gonna have to respectfully disagree that they’re the absolute best move if reliability is your #1 concern. Tbh, I’ve seen a few of those cheaper units overheat or have weird sync issues during long receptions, which is basically a nightmare when you're shooting a wedding. Since you have a $400-$500 budget, I’d seriously suggest looking at the Westcott FJ80 II Universal Touchscreen 80Ws Speedlight. It fits right in your price range and the build quality feels a bit more professional and sturdy on a heavy body like the 5D Mark IV. The color consistency is really impressive for a non-Canon flash, which helps a lot with skin tones during indoor mixers. I’m not 100% sure if the AF-assist is exactly as snappy as the native Canon beam in total darkness, but in my experience, it’s been way more reliable than most third-party stuff I’ve tried. Plus, the UI is so simple that you won't be fumbling with buttons in the dark. Honestly, for the price, you could almost get two and have a dedicated backup, which is the only way to stay truly 'safe' during a paid gig anyway!

Stumbled upon this discussion and i just had to chime in because i went through the exact same struggle! I literally had a mini heart attack the first time I shot a corporate event and my flash wouldnt recycle fast enough lol. After that, I became super cautious and totally obsessed with reliability above everything else. ^ This. Also, if you are really worried about that wrist fatigue but want that total peace of mind, you should seriously look at the Canon Speedlite 430EX III-RT. I ended up grabbing one for my 5D IV and it is a total game changer for my indoor gigs. It is so much lighter than the big 600 series units, so my wrist doesnt feel like it is gonna fall off after a long day. The AF-assist is absolutely fantastic and works every single time even in those super dark reception halls where other brands might hunt for focus. I have never had a single misfire with it, which is the only thing that keeps my anxiety down during the big moments like the first kiss! TL;DR: Grab the Canon Speedlite 430EX III-RT for perfect Canon compatibility, way less weight, and amazing reliability while staying under your budget.

Finally someone says it. Ive been thinking this for a while but wasnt sure.