Features of a complex interface: learnability or discoverability?

I am currently working on a new version of a Saas which clients can use to manage all of the content on their screens, which is going to be quite extended and complex.

To make sure it will be easy to use for the target users, I am conducting frequent user testing sessions every time I update my prototype. However I noticed that the users who are completely new to this kind of system struggle with one particular part of the process, because they don't know (or understand) the mental model behind the service.

My question therefore is: Should I keep making iterations so that make sure that the users are able to do everything they need on their own right from the start?

Or should I invest in designing a good tutorial/interface tour for the new users so that they know how to use the site afterwards?

I am struggling with this and I would like to avoid for our team to use their time to train clients on a too-complicated interface like what is happening with the current version. Thank you to anybody who will answer!