Under normal use, you do not need to preemptively waterproof the tent fabric of your Holtkamper. The tent fabric is naturally waterproof and will remain so with proper use and maintenance.
The most important thing is to care for the tent fabric properly. Do not use unnecessary cleaning agents or waterproofing agents unless there is a good reason to do so. These can actually affect the properties of the tent fabric.
Do you notice moisture on the inside of your tent while camping? That doesn’t necessarily mean the tent fabric is leaking. It’s often just condensation. Moisture can accumulate on the inside of the fabric, especially when there are temperature differences and high humidity.
Good ventilation helps prevent condensation as much as possible. Therefore, make sure there is sufficient air circulation inside the tent, even when it’s a bit cooler or more humid outside.
In short: under normal use, you don’t need to make the tent fabric any more waterproof. Proper maintenance, adequate ventilation, and storing the tent fabric in a dry place are the most important factors for keeping the fabric in good condition over the long term.