1000 total number of users, even if 100 are practising for their "Theorieprüfung" at any given time, I would run that operation comfortably in a single 4 vCPU, 8 GB RAM, SSD VPS. But I experiment with Raspberry Pi and similar low power devices and familiar with the inner life of them and always in a mission to save energy (and the environment).
Whatever, your 10 vCPU, 32 GB RAM, SSD system should be smooth sailing. You need to dig deep in to the real cause of the problem rather than prescribing medicine that are known to have helped others. The specifics are in those documentation and past discussions in this forum.