Sure but the backup happens each day and then gets overwritten/deleted when the next days backup happens (which then deletes the disappearing messages that are expiring express the next backup). It just ensures you have access to any messages that you’re supposed to have access to according to the timers on said messages.

That's not how forward secrecy works. Ciphertext isn't "deleted" unless the key used to encrypt it is also deleted. That's the point of Signal's cutting edge protocol. This undoes all of that.