WSUS 2016 "allows" 3rd party patches through APIs, but it is buggy. Is there another free alternative built in to say Windows Server 2019?
SCCM isn't an alternative to WSUS. It simply adds an orchestration layer on top. It's still WSUS under the hood.
Since MSFT already makes SCCM and WSUS any other service offering would be in competition and that's generally not allowed at MSFT.
However, you can look at third party offerings like Novell ZENworks or build your own.