Another clue: the CiviCRM -> Status page `/civicrm/a#/status` was completely blank 

This helped [it's a cache clear of sorts]

`Administer -> System Settings -> Asset Caching -> Disable`

brought both Actions as well as the Status page back. Then was able to toggle it back to -> `Auto`

For one project I also had to re-run composer civicrm:publish (some bower assets - only some - most were there) - were just not there post upgrade.

Most sites upgraded just fine. Not sure what was/is different about the three sites I ran into this issue with.