Scout is an easy to use web app that can be used aggregate devices across multiple Jamf Pro Servers. It's completely open source and free to use. Scout consists of two main parts: a REST API built with node.js and express, and a client dashboard application. The server is very lightweight and could even run on a Raspberry PI.
Scout has the ability to aggregate devices from hundreds of Jamf Pro Servers, and handle up to ~10 Million Devices. When a server is added, the user specifies a time and frequency to pull data from that Jamf Pro Server. The server spins up background workers for each of those servers at the time specified, and updates the in the Scout database. Anyone with access can also export csv reports or complete basic MDM commands. Scout also supports webhooks, for just in time updates.
The node.js server will need to remain running all the time, but exposes endpoints like /mobiledevices and /computers to get all of the devices in your servers programatically. You are able to use just the server, just the app, or both! The server automatically servers up the web app as well.