Ok, unless you setup a
Trial Date field={{system.Date}} and then have an SFDC formula field to calcualte Days Active, you will need to use Wait Steps:
Trigger: Enter Trial
Wait 10 Days
Send Email X
That will do what you want. I'm not sure why it matters to you if a Wait Step is there. This should be a triggered flow, not a batch.
Yes, IN PAST is not the operator you need. There may be an idea to help on this, so be sure to vote it up.