Replying to my own question, just in case someone has the same issue.
I found that the best way to let user import data is to have them email a mailparser.io inbox and with the proper filtering import directly into Bubble via an API workflow.
And I used this reply by @NigelG to figure the details out. Email to Create a thing / update a thing?
Thanks Nigel! I am doing mailparser.io to Bubble, skipping Zapier.