I got the answer on my crosspost on Grafana Forums.

STR_TO_DATE(CONCAT(YEAR(`Timestamp`),' ', WEEK(`Timestamp`, 3),'1'), '%X %V %w') AS Week

This did the trick. It is a compromise that works.

Unfortunatelly, it is not ideal, because the format of the original query would be way better for visualization, but I guess that is where Grafana is as of today.

Also, the fact that the error message goes “Data is missing a time field” instead of “Incorrect time field format” is confusing.