Try Kdeconnect, you can set up multiple kdeconnect clients and send files between eachother including mobile clients, it works over VPN too.
If you are looking for a screenshot sending and hosting see this
https://github.com/SergiX44/XBackBone
There is a Linux bash script to send screenshots and files to it (see the page), you probably could use it under Mac. Ot find a more elegant way to use its REST api under Mac