The only existing trigger for customers is "Customer created".
A solution would be to find an app (or a developer who can build one) which would implement a new custom Trigger "Customer last purchase x days" or something more specific to your needs. Such an app would need to check on schedule when your customers have made purchases and fire off the trigger in Shopify when appropriate (or the app could untag them directly through the app)
Full list of existing triggers can be found here:
Rough technical break down of the task (that you could pass to a developer):
- Setup a new customer app in Shopify
- Setup a scheduling mechanism ( cron job, azure scheduler, etc. ) to poll customers from your store using app credentials every day
- Untag all customers who have not made a purchase in x days (or fire off a custom "Customer has not completed an order in x days" trigger in Shopify Flow)