Honestly, making that jump from the 5D to mirrorless is a bit of a shock to the system. I went through the same thing a few years back. The EVF is basically always on and just drains everything way faster than the old optical viewfinders ever did. Since you are doing a wedding in Seattle, you really cant afford to run out of juice mid-ceremony. In my experience, you have two real paths with that $300 budget:

• Canon LP-E6NH High-Capacity Li-Ion Battery: I've tried many off-brands over the years, but the official NH version is the way to go. It gives you the best burst rate and allows for in-camera USB-C charging. Pros are the reliability and communication with the camera, con is just the price tag.
• Watson LP-E6NH Li-Ion Battery Pack: These are decent third-party alternatives if you need to bulk up your kit on a budget. They're much cheaper than Canon. The con is that sometimes the camera wont show the exact percentage accurately or you might lose some high-speed shooting modes.
• Canon BG-E22 Battery Grip for EOS R: This was a total game changer for me. It holds two batteries and makes the camera feel way more balanced, especially when using heavy EF glass with the adapter. For weddings, having that vertical shutter is a lifesaver for portraits. If I were you, I'd grab one extra official battery and the grip. It keeps you from swapping cells every hour and makes the body feel more like the 5D you are used to. If you still have cash, grab a SanDisk Extreme Pro 128GB UHS-II SDXC Card because the write speeds on older cards might lag your buffer during the ceremony... nothing worse than a frozen camera when the kiss happens.

> the battery life is tanking way faster than I thought. Yeah, the mirrorless transition is definitely a learning curve for power management. I saw this earlier today and wanted to chime in because I made the same jump from the 5D too. If you still have some cash after batteries, you should definitely look into the Canon Mount Adapter EF-EOS R with Control Ring. Honestly, it makes those old EF lenses feel brand new since you can map the ring to your ISO or exposure comp. Its been a game changer for me. Also, for a wedding where youre on your feet all day, Im super satisfied with the Peak Design Slide Camera Strap Black SL-BK-3. The stock Canon straps are basically trash tbh and this one distributes the weight of that heavy EF glass way better. It works well and I havent had any neck pain since switching. Feel free to ask about the autofocus settings too because thats another rabbit hole.

Same here!

bump

Regarding what #2 said about "> the battery life is tanking way faster..." - honestly, its such a shock coming from the 5D world. In my experience, those old batteries lasted forever because they werent powering a mini-computer screen inside the eye cup 24/7. Since you're doing a wedding, you really need a backup for your backups. Tbh, I'd suggest looking into a solid power bank. Just get any of the high-capacity ones from Anker. You really cant go wrong with them for PD charging on the fly. I've tried a bunch of off-brand power blocks and they usually fail when you need them most, but Anker is basically the industry standard at this point. Also, if you find yourself needing more light in those dark Seattle venues, just grab some flashes from Godox. Their whole wireless system is way more flexible than the native Canon stuff for a fraction of the price. Just stick with a reputable brand like that and youll be fine for the big day.

Wow ok that changes things. Gonna have to rethink my approach now.