Hmm, I believe the below might be able to do this. By being paid the subscription should become Active so you should be able to condition based off the subscription state.

Automatically send message at following date: 5 days after
Relative to this date of the plans below: Subscripton's subscription date
To which plans this date condition applies: SELECT_PLANS_HERE
Must be in state: Registered Unpaid
But do not send message if it's later than: 6 days after (suggesting this to avoid spamming them as ideally you'd trigger processing daily)

That should ideally send your mailer if 5 days after their subscription date the select plan(s) are still unpaid.