Like someone mentioned, that SD slot is basically a bottleneck that ruins the whole experience. Honestly though, I disagree with the folks saying you need a battery grip. I had one for a season and it just made the camera way too heavy for a long wedding day. My shoulder was wrecked by the cake cutting and it didnt really solve the core issue. Unfortunately, this camera just isnt as efficient as it should be for the price, and adding bulk doesnt always fix the frustration. Here is what I would do with that $500 budget to actually make it reliable:

• Stop using those cheap knock-off batteries. They're totally unreliable and the power curve is a joke. Grab a couple Canon LP-E6NH Lithium-Ion Battery. They work in the Mark IV and actually hold their charge during long bursts.
• You absolutely have to use the CF slot for your primary recording. The SD slot is just too slow for 4K video or fast weddings. I have had great luck with the Transcend 128GB CompactFlash Card 1000x and it handles the data way better than any SD card ive tried.
• Since you are in Seattle, get a Peak Design Shell Camera Cover Medium. Those plastic bag style covers are kind of a mess to work with and this one actually stays out of the way while keeping the body dry. Also, check your settings... if you have Dual Pixel RAW turned on, your buffer is gonna choke no matter what card you use. Its a neat feature but basically unusable for fast-paced events. Good luck with the next gig, hope it stays drier than the last one.

Jumping in here because that buffer issue is basically a hardware bottleneck. Like someone mentioned, storage is a huge factor, but specifically, the SD slot in the 5D IV is actually capped at UHS-I speeds. This means even if you buy the fastest SD card on the market, it wont write faster than about 95MB/s. For 4K video or high-speed bursts, you really need to utilize the CF slot which supports UDMA 7 for better throughput. If you want to fix this within your budget, here are the technical essentials:

• Lexar Professional 1066x 128GB CompactFlash Card provides up to 160MB/s read speeds and handles high bitrates much better than SD. Its usually a bit cheaper than the SanDisk equivalent but just as reliable in my experience.
• Canon Battery Grip BG-E20 is a solid investment if you can find one used to save some money. It lets you run two batteries simultaneously, which effectively doubles your shot count and improves the camera's balance with heavy glass.
• Think Tank Photo Emergency Rain Cover Medium is basically mandatory for Seattle weddings. Its cheap insurance for your gear when the weather turns. Tbh, generic batteries often have inconsistent voltage curves that can cause the camera to misread the remaining charge. If you move your primary write to a high-speed CF card and just use the SD card as a low-res backup, you should see those lockups disappear.

I once missed a bride walking down the aisle because a cheap battery died at 20% and my slow SD card locked the buffer... it was terrifying. Dont trust generic power, so make sure to only use Canon LP-E6N Li-ion Battery Pack. I would suggest getting SanDisk Extreme Pro 128GB CompactFlash 160MB/s tho since it handles the data load way better. TL;DR: Fast CF cards and genuine batteries are the only way to stay safe.

Yep been there done that. Can confirm everything said above is spot on.