When activating Google's Gmail logs to BigQuery, will the system export all historical data or will it only export mails arriving from today onwards?
The reason is that we want to analyse all email data easily. As such, an export to BigQuery appears more convenient than crawling all API endpoints for all users. However, if this is not going to contain historical data, it will not be very useful.
If this isn't the case, what would be the most elegant way to export all Gmail data in a "data way", not crawling the API endpoints?
According to the link that you posted:
Note: Email logs created before you set up Email Logs in BigQuery can't be exported to BigQuery.
So, it seems that it can't be of use for you. Have you checked the Workspace data export functionality? It exports data from all Google Workspace, Gmail being one of them.
You can see here what's included.