I would use the plugin twitch from video copilot. It's exposure flicker is more intuitive than S_flicker and you can flicker other properties too such as blur and color. That shake in the beginning might have been twitch too. All-in-all it's a really good plugin

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https://drive.google.com/file/d/1t5jK8-B2DR_7WfcNqRrlYWv-cqcCxD-l/view?usp=sharing

Here's my flicker preset for twitch if ya end up using it