*Related posts based on categories, tags.
*Can be replaced and sort of related posts at admin screen.
*I can be added as an add-on another related posts algorithm.
You can make addon to use Addon Template.
You can use JSON REST API Endpoint. Require plugin JSON REST API
http://example.com/wp-json/posts/%post_id%/sirp_related/
http://example.com/wp-json/sirp_related/%post_id%/
http://example.com/wp-json/sirp_related/%post_id%/?filter[num]=5