How can I redesign a request for quote workflow with the most user-friendly structure which is quite easy to navigate?

My concrete question: How can I redesign a request for quote workflow with the most user-friendly structure which is quite easy to navigate?

I am struggling the last few days how to best possible create the structure and therefore the navigation for our tender plattform.

The tender plattform works the following way:

1. The potential buyer creates a request for quotation ("Angebotsanfrage") with one or more product positions. Each position is a customized item with about 30 fields (there is a separate step-form used to fill out the information). In addition he adds some general information about the request name and logistic information such as the delivery address.
2. The potential buyer submits the request for quotation. After the submission an additional 4th tab appears which is called "Quotes" where all incoming quotes from sellers are displayed.
3. Several sellers which match according to their profile the technical capability to receive an e-mail notification and an notification in their system inbox.
4. Each seller decides whether to bid or not to bid. If they bid, they submit via the system a quote.
5. The potential buyer receives from each seller who bids one quote. He can see, review and decide on them when he navigates in the request for quote ("Angebotsanfrage") menu point, selects the relevant request for quote.

I struggle especially with the steps which I describe from 1.2.1 to 1.2.2.2

The user story and the "drill-down" navigation works as follows;

1. User Login - Navigation Level 1
    1.2 Request for Quote - Navigation Level 2  
        Create new Request for Quote - Navigation Level 2
            1.2.1 Produktinformation ("Product information"); Tab)- Nav. Level 3
            1.2.2 Neue Position - Navigation Level 4
                1.2.2.1 Auswählen des Produkt - (Dialog) Navigation Level 5
                1.2.2.2 Menge & Bestellart - Navigation Level 4

The first Wireframe shows Level 3 (Nav. element 1.2.1)

The second Wireframe shows Level 4 (Nav. element 1.2.1). The font is red, because this approach is my current one and I think is really bad, but I just cannot think of an alternative.