I’m also wondering what to use in place of $post_id when using save_post with an options page.
I found the following on a question related to acf_form and options. Not sure if it applies, but I’m going to try it:
Just set the post_id param to ‘options’.
I found it on this post: Using acf_form with options plugin.
My understanding is that all options pages use the options identifier, so I don’t know how you would distinguish between multiple options pages. Perhaps you could test for a unique field for each options page to determine which is which.
I will test using options in place of $post_id and let you know what I find.
