Real-time dashlet of contacts without certain activity
If the smart group approach is working other than the update latency, you could reduce the Smart Group Cache Timeout at Administer > Customize Data and Screens > Search Preferences.  Beware that it is likely to increase load on the system, but that may be worthwhile in your case.

See https://docs.civicrm.org/user/en/stable/organising-your-data/smart-groups/#smart-group-caching