Transitioning from a python script for data transformation to BigQuery
Possible Solution:
Assign a UUID to each email/user instead of an auto-increment ID (first making sure email does not already exist else use the existing UUID).
UUIDs can be created anywhere - not necessarily in DB - and are almost guaranteed to be unique accross time and space.

For example, the number of random version-4 UUIDs which need to be
generated in order to have a 50% probability of at least one collision
is 2.71 quintillion, computed as follows:
$${\displaystyle n\approx {\frac {1}{2}}+{\sqrt {{\frac{1}{4}}+2\times \ln(2)\times 2^{122}}}\approx 2.71\times 10^{18}.}$$
This number is equivalent to generating 1 billion UUIDs per second for
about 85 years. A file containing this many UUIDs, at 16 bytes per
UUID, would be about 45 exabytes.

Note: User Email can also be used as unique ID and makes everything easier.