Kia ora Melissa,
We use the Site logs report, it's not ideal but better than nothing. You'd have to check that it has a line for workshop attendance that you can use.
These posts have a bit more info about using them and also the amount of time they are kept changed.
Totara Learn Open Discussions - Learners retaking courses and completion date
Totara Learn Open Discussions - Bespoke report to show latest completion dates
Ngā mihi,
Larry