The way the tunnel got filled up with the reverb zones doesn’t look very beautiful. So I looked this feature up in the internet and found this article: https://answers.unity.com/questions/58585/non-spherical-reverb-zones.html.
There are two other solutions given:
The easiest way is to make one big reverb zone around the cave and add trigger-boxes in the entrypoints which activate or deactivate the reverb zone.
The other solution said that the reverb zone is not specialy made for the inside of a room but more for the transition between two rooms. During the transition you can setup your stepsounds to make it sound like you want to and the soundeffects inside the cave should already be preset to the environment.