Targeting users who have done only one action in a flow?

We have a custom metric set up “Trial Started”, where a user activates a trial of one of our apps, but this logic can apply to e-commerce orders too.

I want to create a flow that sends an email to users to started a trial for Product X only, so if they’ve started a trial for Product Y or Z ever, they will not be in this flow.

So far, the only way I can see to do this is the use the “What a user has not done” filter and create a list that includes the SKU of every other app of ours, like this:

But this would be awful to maintain.