Hi @CFitzgerald,
When you switch this on, only enrolling contacts who meet the criteria from that point on, that shouldn’t be an issue. Ignoring existing contacts, the workflow would trigger the moment it detects an email. This wuld be after an email send from that template. From a first glance that should be activity-specific enough? (Unless there’s an additional requirement?)
This becomes only an issue if you want contacts who only received this email. Or whose first or last email this was, for example.
If re-enrollment is a concern (activity properties don’t allow for re-enrollment), you would have to switch to a list as an enrollment trigger. Lists can be used for re-enrollment. Your list criteria would look like your workflow enrollment triggers, only that you’re additionally limiting the activity date to be within, say, the last 24 hours. This means that contacts will leave the list automatically and re-enter when they’re eligible due to a new send.
Hope this helps!