I would suggest going with the SmallRig Cage for Canon EOS R5 Mark II 4734 for now. Honestly, you might want to consider the extra weight before you buy the Tilta Full Camera Cage for Canon EOS R5 Mark II with Cooling System because weddings are long days. Be careful about adding bulk you dont actually need... unless youre shooting 8K all day, the basic cage is usually enough.

Quick addition... if you want quality, the Kondor Blue Canon EOS R5 Mark II Cage is solid and has built-in tools. For something lighter, the 8Sinn Canon EOS R5 Mark II Cage has a better handgrip feel imo. Look at the CineD thermal database for R5II tests; usually, fast CFexpress cards help with heat more than a fan does. TL;DR: Get Kondor Blue for durability or 8Sinn for ergonomics. Fans are overkill for weddings.

TL;DR: I am in the exact same boat and honestly it is stressing me out big time. I have been dealing with this exact same problem for like a month now and still cant find a straight answer. In my experience over the years, reliability is everything, so the heat thing scares me, but I am also looking at the price difference and wondering if the extra money is just for peace of mind or a necessity. I have been stuck in this loop for weeks and still havent pulled the trigger because I am worried about making the wrong call. It is so frustrating because I need to get moving on it for my own upcoming shoots but I am just as paralyzed by the weight versus cooling debate as you are. Still nothing has really cleared it up for me yet and the clock is ticking.