Very interesting.

"But all in all, we take aboard around 20% of the applicants who started the recruitment process."

What is the % that take the survey and move forward?

Since the 'recruitment' process starts after passing the survey.. Have you given thought to analyzing the survey results of the 80% that don't make it, and see if you could better adjust the survey or conditions to weed out more undesirable candidates earlier? It seems quite resource intensive on your side to perform this process.