Since any given flow instance can only run for 30 days I would do this with three different flows and some data source to record status.
Flow #1 would be similar to what you have now but without the delay. Instead when you flag the email it would record the Recipient and Subject in some persistent data source, like SharePoint. I would also include a status field.
Flow #2 would use a recurrence trigger. It would run at whatever interval you want to send reminders. It would get a list of emails from the data source that show a status of no-reply and send out reminders.
Flow #3 would use a trigger for when an email is received. It would check the sender and subject against the data source to see if its a reply to an email added in #1. If it is it would either delete the email from the data source or flag the status as Completed.
That pattern will scale to as many emails as you want to flag and make it easy to see which ones have received replies and which haven't.