Solution for remotely managing 100+ WAP each on their own network? (AirBnB)

I manage 100 (and growing) AirBnB units throughout the midwest, and WiFi issues are taking more and more time. Currently, each unit uses the modem/router supplied by the ISP (most Comcast and Charter), with the default settings other than a custom SSID name & password. I'm looking to implement a scalable solution that allows me to:

1. Monitor network outages
2. Monitor devices and usage
3. Configure router settings / reset pass
4. Diagnose and fix misc issues

I believe the first step is identifying the software solution, then replacing the current hardware with compatible devices. Can I get some direction of what I'm looking for? Googling has led me to the Unifi products, but I'm not sure if that's what I'm looking for.

Additionally, I prefer open source / self-hosted to save on costs - I'm thinking the router flashed with custom firmware (is this still a thing?) that can be controlled via an admin panel on a webserver.

Thank you for your time!