What exchange do your coins come from? Let's see maybe there is an issue between that exchange and Coinbase.
Are you in the UK right now?
You are living in a very strict AML law since you transferred a very big amount of coins it will take too long to verify if you didn't submit what they ask.
What I think is to try to submit what they ask you said their email is in your junk why not try to submit what they ask even if it's late?
Use the contact email they provide from your email to submit what they ask.
My coins has built up over time, it wasnt just one large transfer. I cannot submit what they ask for because it asks me to login first if that makes sense.