How to build a complex page structure

Let’s say i need a page structured like the image below.

There is a frontpage and from there i can choose one of the sites partners. Every partner gets a additional “partner page” which has to be editable from within the dashboard. Below the description of the partner would be a list of projects the partner has done. Every project is also fully editable.

Best case scenario would be a permalink structure like this:

• Partner: www.example.org/partner1
• Project: www.example.org/partner1/project-name

I dont know which approach is the best for this, should i build categories or taxonomies? This all seems tedious because there will be more than 100 partners and their projects.

I have build a lot of themes but never with a structure like this, so what is the best way to solve this ? (Categories, Taxonomy, CPT, some Plugin …)

I would not use WordPress pages nor articles for this.
Simply, create a new custom post type called “Projects”, with a plugin like Types, then create a new custom taxonomy “Partners” and associate it with that custom post type.
Clean and easy structure.