Negative pressure

if you use an air filter for smell, negative pressure is essential to its effective function... if you don't care about smell, then you don't necessarily need negative pressure. Simply adjust the fans so that as much is entering as leaving and the walls will no longer bow.

no. Its fine, and if anything neg pressure is wanted to keep the smells in. adding in more vents can help the air flow a bit easier or even removing the filters on the exhaust fan if you have any like carbon. filters are not really needed if you are venting outside.

Das muss so sein, soll ja kein Geruch nach draussen.

Your setup is fine. Negative pressure is expected and even beneficial. Unless the walls are collapsing onto the plants, you don’t need to change anything.