design and development workflow

Question

We're developing a set of websites, complex ones with a lot in common, and the company have decided to use WP as a base CMS.

As in every site, there is a design process, followed by HTML, then development/CMS integration. However, I'm wondering how would that work with WP?

Do companies design all templates, then HTML, and then theme and CMS development, or do they finish the CMS development (which in this case a layer of custom plugins extending WP), then create individual themes that works on it?

As time is crucial and many people are involved -which raises the cost- , I would want to raise efficiency of the team and make design and dev teams work in parallel.

Any idea of what the sequence should be are appreciated.

By the way, we have common features amongst the sites and then we have site-specific features, which you can think of as theme specific features