Hello! Thank you for your guidance. I had actually already tried using update_post_meta but it wasn’t saving as an additional field in ECP. It was just saving as custom meta for a wordpress post (my understanding is that these aren’t exactly the same thing). This would have been totally fine except anytime I updated the post I would lose the value of the custom meta.
It turns out the filter I was using for save_post (for ECP posts) wasn’t checking for meta and therefore was erasing any saved meta values. I just needed to modify it and now it works!
Thanks again for your help!