I have done similar prototyping in docs, whiteboards, CSV files... The flow is to focus on the data and what are the constraints(rules) on updating the data. The prototyping of api's, screens can come later off of a firm foundation of data, calculations and rules for creation and change.

Show me your flowchart and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won't usually need your flowchart; it'll be obvious." -- Fred Brooks, The Mythical Man Month (1975)

Not the OP, but I’ve tried this before too. A huge benefit is that the everyone at the table understands excel and isn’t intimidated during the conversation. Understanding the domain as an engineer is the toughest, most important, and most rewarding part of DDD imho.

I actually like to screen-share or project this: https://start.jhipster.tech/jdl-studio/. You can update this real-time and see your ERD real-time.

Check out the relational data modelling capabilities in the Power Pivot Diagram view. It helps you graphically see the relations between basic excel tables.