wp-insert-post execute from server?

I wanted to ask for help.

I am working on a project to develop a program to publish a post to many wordpress sites that aren't on a multisite network but are all on the same server.

the requirements for this program is that it does not take delay for publishing so the post should be sent once onto the server and the server will have to add the post to many sites and the program also needs to filter the publishing of the post to sites with the requested categories.

I Have started to work on it. i found 2 solutions to adding a posts to a wordpress site:

1. wp-insert-post and other functions in the wordpress documentation
2. xml-rpc

for now i understand the program will be slipt into 3 parts:

1. an interface where the sites with the similar category will be selected to the post.
2. a program on the server that receives the publish request and the chosen sites to submit it to.
3. a program to publish to the website.

if anyone has any tips and ideas to help or has worked on a similar project please help.