My understanding is that post SEO strength decays faster with time but is stronger than a page in the first instance. Something that is important and has relevant persisting phrases that you always want to rank well for in search engines should be on a webpage.
Originally posts were mainly for blog entries.
Pages like About and Contact that don’t change often are better as web pages.
On my sites, I post when something is topical for the here and now. Eg date or current event related. Even informational content that is not cornerstone content.
You can convert webpages to posts and posts to web pages manually or use a post-type WordPress plugin to convert.
Remember when posting to create links from your post to existing other posts and web pages, especially cornerstone pages.
For both posts and webpages interesting related photos hold attention to your post/page longer which is good for SEO.