Generating new VAPID keys can invalidate existing subscriptions until users subscribe again.
Send a one-off push notification through the default delivery API without changing the saved schedules.
Save as many schedules as you want, then preview or send any one of them through the default delivery API.
This page saves schedule data through
config-api.php?action=save and sends matching notifications through
delivery-api.php?action=custom. Run API/schedule-worker.php from
cron to process saved schedules at the right time.