Responsive Design – Vast Number of Screen

I'm currently working on a very complex web project. The site is vast, over 300 screens. We are redeveloping an existing product so customer familiarity is a consideration.

The navigation of the old site is so hard there is a screen search facility to bypass the site navigation. To my mind that just feels like admission that their navigation is completely broken.

The pages vary from very big forms, pages, wizards, wizards with complex forms data-vis and large tables of data and no doubt stuff I haven't even seen yet.

The idea was that this redevelopment would be mobile first. The problem is the team have been designing for desktop and have pretty much replicated the old navigation.

The question is, is it even feasible for a site of such complexity to ever be mobile first? Are there any guidelines for such complex navigation?

The site is hard enough to use on desktop as to only be scraping by as desktop friendly, let alone mobile friendly or first. I know I personally wouldn't even try it on a mobile device.

At some point I have to provide some advice on the mobile strategy and I'm already seeing a suite of at least 3 apps. Even building this natively in a single app would be challenge I wouldn't want to take on.