Back in January I was going through the same throes vis-à-vis El Capitan — see "Downloading Mac OS" thread. Urquhart was most helpful in his post #47409 (2018-01-05) which follows (somewhat edited):

Originally Posted By: Urquhart
... a fresh copy will be the latest iteration: 10.11.6, but without the later security updates.
If ... an older installer earlier than 10.11.6, you’ll need the 10.11.6 Combo Update (1.5 GB) that'll work on any version 10.11.x. You can download it manually with any browser or use the App Store app.
I believe the security updates include all previous security updates (cumulative), so you’ll only need one: Security Update 2017-005 El Capitan (630.4MB). You can ... download it manually with any browser or use the App Store app.

Subsequent to that there were several threads with dealt with a number of El Capitan issues.