After a lot of trial and error, I finally found a solution to how to send emails automatically on a defined schedule from a model-driven app. This blog post helped me to find the solution:
The main thing here is to realize that you need to use the ‘perform a bound action’ -step in the flow:
My approach is to have a custom field on the email entity in which the end users can enter the DateTime for the mail to be sent, then my power automate flow runs once an hour to see if there are any scheduled emails to be sent and will perform the bound action.
Note to self: just updating the email status from draft to pending does not trigger the send.