Since Apple today re-issued the 2019-004 security update I thought I'd re-install it, but this time directly from the appstore. Firstly it went through the "Installing software" part, which finished in about 10mins (despite it calculating 20mins), then it went through "Installing software" again, but this time in didn't progress beyond a couple of mm on the progress bar. After a while I just gave up and hard-rebooted.
What I discovered in the install log after rebooting was 2 things.
1. the replaceRecovery script failed at the hdiutil attach command (the problem I'd always had with the un-edited script in the standalone updater)
2. the reason it didn't progress through the second part was that there was a whole host of messages saying (once a second)
Software Update[4314]: Not yet started snapshotting. No progress will be shown.
So despite others having success in installing security updates directly from the appstore, it doesn't work for me. FWIW, I'm using HFS - don't know if that's the issue.