Building on the earlier suggestion, im really satisfied with Peak Design Slide Camera Strap. It works well and saves my neck. TL;DR: Get a comfortable strap for long wedding days.

^ This. Also, be careful with slow media. Assuming you mean the R6 Mark II, that thing moves serious data. Tbh, using old cards is a recipe for disaster during the vows.

• ProGrade Digital SDXC UHS-II V60 Gold 256GB is the bare minimum imo.
• Lexar Professional 2000x 128GB SDXC UHS-II V90 if you want total peace of mind. I'd suggest staying away from anything V30 or slower.

Honestly, I've been in your shoes and it's a total panic when you're missing the basics right before a big event. I remember my first time shooting a rainy Seattle wedding... it was a disaster because I tried to cut corners. I unfortunately used some off-brand batteries once and they just died halfway through the toasts, so don't make my mistake.

• Memory: You need serious speed. I had issues with lag on basic cards during fast moments. Pick up the SanDisk Extreme PRO 128GB SDXC UHS-II V90.
• Power: One battery won't cut it for a full day. Get at least two Canon LP-E6NH Lithium-Ion Battery so you don't go dark.
• Protection: Since it's Seattle, you definitely need a Peak Design Shell Camera Cover Medium. My last gear wasn't as good as expected in the mist and the buttons started sticking.