how do i get past the password requirement to back up and restore my 8plus?

The bottom line here is that if you used a password with the previous backups of the phone, you must enter that password to access the backup.

Unfortunately, if you cannot remember the password that you used with the backups, you won't be able to use the backup. See this Apple Support document for details and confirmation.

If you can’t remember the password for your encrypted backup - Apple Support

The unfortunate detail.....

"You won't be able to use previous encrypted backups, but you can use the Finder or iTunes to back up your current data and set a new backup password."

You do not remember your encrypted backup password, I can understand.

You do not remember having created an encrypted backup, I can understand this also. (This is not true in your case)

This means the iCloud Key chain was checked while creating the encrypted backup because by default it is checked.

Look for it in the iCloud Keychain.

Did you un-check the keychain while creating the encrypted password (By default it is checked) See the pic hereunder.

If you had un-checked then the only option that you have is, keep trying various passwords as there is no limit on the number of attempts.

You may have been given this option as given in the pic below whien you had first synced your iphone to iTunes/Finder. You may have checked "Do not warn me again"

You may have forgotten that you had entered a password. --> About encrypted backups on your iPhone, iPad, or iPod touch ...

You set a password to encrypt the backup. Without that password, you can not restore that backup.