conditions , subscription contracts, and Shopify API...Oh my...

[RendezvousScent]

Hello fellow business owners! I wonder if any other subscription services have solved this quandary: 

I would like to send follow up email flows to my customers half way through their subscription to one of my Scent Journey products, as a way to check in on how they are liking their Scent Journey. 

The catch is I allow customers to SWITCH their Scent Journeys, switch into a collection of other monthly products, and thus only want this check in email to trigger IF the customer has been SUBSCRIBED TO THE SAME SCENT JOURNEY. That way I can get an accurate understanding of their experience with a single journey, and not one fragmented between 2 or more journeys. 

And if that can be figured out...How to send a sister email at the 6 month completion of a Scent Journey if they've stayed on the same one the entire time.

I would be so grateful to anyone who can help me understand how to solve this. My website is Rendezvous Scents for anyone who'd like more context... 

THANK YOU IN ADVANCE! 

[paul_n]

I'm not sure that this request is related to Shopify Flow. Can you clarify?