I agree with Denin and that you will have to do this with a notification and not a Subscription. Notifications can go to any email address, but subscriptions can only go to users in your app.
So what you do is you create a formula field on the record that calculates to be true if today is the day that the reminder needs to be sent. Then you have a scheduled pipeline which does a daily search for these records due to be sent and what I typically do is update a date time field to the current date time in the pipeline box. {{time.now}}, thus triggering the Notification.
I forget offhand if the new custom emails will send to non-user email addresses, but certainly if you create a legacy style notification and right at the very top say that it is an OPEN notification, then in the box as to who gets notified, you can actually carefully hand type the field name holding the email address. For example.
[my email recipient]
And in that same box, you can also list vertically in the other hard coded email addresses you want to receive.
so it will look like a vertical list in the box
[my email recipient]
[email protected]
[email protected]