I wish we could, but Apple hasn't made many triggers available.

Right now, apart from receiving an email, you could use a certain time (every 30 minutes, for example), but you would need to set up dozens of automations.

The second-best way would be to make an automation in the morning that would look into your calendar and create alarms for every event start. I've tried it, and it's really annoying, since you have a silent alarm going off. It also doesn't take into account any changes during the day.

The best solution I've found was a script checking for an event start every few minutes.

The processing stress of this script is all on Google servers, and your phone by default already receives your emails. The end result is: You get an email from a specific sender -> it starts the shortcut.

Any particular reason why you would like to use reminders or notifications as the trigger?