Carbon Fields is a library that will appeal to developers and power users. It is a free, code first solution. It lets you create Custom Post Types, custom taxonomies, metaboxes and custom fields. You can assign fields to posts, Custom Post Types, taxonomies, users, and generally for site options. You can also create Gutenberg blocks using PHP.
You can install Carbon Fields by downloading the plugin from their GitHub page or through a Composer command. Details in the Carbon Fields docs.