Regarding what #1 said about "I shot a dark barn wedding last fall", my Godox V1-C Flash for Canon unfortunately overheated mid-dance. The Yongnuo YN600EX-RT II for Canon was better. TL;DR: Use Flash Havoc.

I shot a dark barn wedding last fall and the wood just eats your light. I've found the Godox V860III-C TTL Li-Ion Flash for Canon is the best bang for your buck on the 5D4. Be careful with those V1 round heads; they can be finicky. I'd suggest sticking to the traditional speedlight shape to save cash and avoid those connection hiccups people mention.

honestly i tried to go the budget route for a low light shoot recently and it was such a letdown. the gear just couldnt keep up with the 5d4 and i missed some key shots because of the recycle time... super frustrating and honestly kinda embarrassing. for that barn wedding, id probably play it safe and grab the Canon Speedlite 430EX III-RT instead. its well within your budget and wont give you those weird connection errors that the cheaper brands always seem to have. one quick tip though... dark wood absorbs so much light that your flash is gonna be firing at full power constantly. make sure you have a couple packs of Panasonic Eneloop Pro AA Rechargeable Batteries ready to go or you will be waiting forever for the flash to recycle mid-dance.

just saw this thread and ngl i am in the exact same boat right now. i have a huge barn wedding coming up next month and my current lighting setup is giving me major anxiety... its the worst feeling when you are staring at a dark venue and dont know if your gear will keep up. honestly though, since we are both looking for that 5D IV reliability on a budget, i have been doing some deep dives into the technical specs to see what actually holds up. here is what i am looking at for my own kit:

• Westcott FJ80 II Universal Speedlight is probably the best bet for that price range. it hits around $330 and the thermal management is technically superior to the V1... in my experience, it doesnt throttle nearly as fast when you are firing off shots during the reception.
• Godox PB960 Propac Power Pack used with a compatible cable. over the years i have realized that pulling power from an external pack keeps the flash head cooler because the internal batteries arent generating as much heat inside the unit.
• also make sure you are using the latest firmware on the body. i noticed some weird lag on my 5D IV until i updated to the latest version, especially when using third party triggers. it is definitely a struggle finding that sweet spot between cheap gear and the $600 canon stuff. let me know if you find anything else that works because i am still hunting too.