What products would you recommend for centralized management of endpoints outside of Fleet/Kolide? Needs to be free/open-source. I know there is a free option for Fleet, but I'd like to see if there are other platforms that have better functionality.
Kolide is not a free/open-source tool. It is a commercial SaaS. (The agent side is OSS, but that’s not what you’re asking for.)
Kolide’s Fleet is no longer maintained by Kolide, and is now FleetDM.
SecurityOnion also exists, though that might be fleet under the hood.
There’s Wazuh. But I don’t know anything about it other than that people ask questions sometimes
You can also write your own. The TLS protocol is pretty simple. But of course, that’s a can of worms