I think this is a super old thread, but I think it’s still relevant.
Here’s how to do it with a filter:
https://kellenmace.com/advanced-custom-fields-metabox-priority-for-top-above-publish/
It would be cool if “side-high” was a selectable option in addition to “side” but I’m guessing everything is focused on Gutenberg-land now and I’m not sure how this applies there.
