First off, take a deep breath. Using a 2018 card on a modern high-resolution body is definitely why your buffer is choking. Those old cards are simply too slow for the massive data files your new camera outputs, especially during burst shooting. Unfortunately, your hundred dollar budget is a bit tight for the absolute best gear right now, but we can still make this work safely for the weekend. I personally had issues with cheap off-brand cards corrupting files when pushed hard, so please stick to reputable names. For your budget, I strongly recommend picking up a SanDisk Extreme PRO 128GB SDXC UHS-I V30. It is reliable, sits right around your price point, and will handle standard bursts much better than what you currently have. Just make sure you format it directly in the camera before Saturday!

Omg I have this exact same issue with my gear... been dealing with it for like 2 months now and still nothing works when I try to shoot fast bursts! It is literally driving me insane because no matter what specs I look at, I cant find a clear answer either. Tl;dr we are both completely stuck in buffer hell.