We are also adaptive in our design approach. Where we find the conditions, we work in Agile (Scrum) basing the customer/supplier relationship on maximum trust.
In other cases, when a lump sum quote is requested, we work in waterfall. We usually use common sense by adopting hybrid approaches.
In any case we envisage the initial drafting of a descriptive project document, with assumptions, requirements and what is generally defined as the project perimeter.
Then, a project plan is drawn up - call it Gantt, Release Plan or Roadmap if you prefer - with the macro-activities for carrying out the same and the ongoing possibility of requesting changes (CR - change request) to be examined together with the Company.
Therefore the project plan as well as the project document are living documents, subjected to physiological variations during progress, called versioning or revisions.
At the same time, in an "Agile" way, a jour fixe is foreseen right from the start (what in Agile is the retrospective of the sprint, what for your grandmother is "the periodic meeting") for progress and alignment on the project, of approximately 1-2 hours every 15 days initially, and then shorter and more frequent as you progress.