Since you have Session-Based Test Management, you can use your test sessions data to show your team's testing capacity.
You can plot the number of sessions your team did throughout time. Downtrends may intigate your team is having problems to keep a steady pace or that your testability has improved, demanding less test sessions for the same ...
Productivity is to be measured in terms of both a. Quality and b. Efficiency.
Defect density: (Number of defects found in production or by end customer) / (thousand lines of new code written).
The lower this number, the better the quality of your testing.
Defects found: (Number of defects found in the year, bifurcated by severity).
This is ...
How can I measure in detail how successful we have worked?
While there are many useless metrics that could be cited her, IMHO there is only one (semi) useful metric of success... number of bugs that have made it into the wild.
If the answer is none, then the team has done its job. Successfully.
If the answer is lots, then the team has not done its job.