That isn't a Backdoor Roth.

The delay makes this a Regular Roth Conversion. Even a "backdoor" would not be "backdoor" if the person also already had nonbasis Traditional IRA, because what you described would be taxed pro rata to basis.

"and is now trying to fill out 2020 Form 8606. I don't see where to enter the amount for line 2 (total basis in traditional IRAs)."

It's just a Conversion; nothing fancy.