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.