I don’t think this will work.
Deleted attachments that are still within your revision history (and thus could be restored in a snapshot or restored from the trash) still count towards your attachment limit.
If you use only native Airtable automations, triggering an automation based on a specific time stored in the record is very difficult and imprecise.
Why do you need the file to be an actual attachment in Airtable?
Are you open to using a third party integration service like Make.com that can better handle time-based triggers and also have that service retrieve the file so the file doesn’t have to be an Airtable attachment?