> An open office plan might well let you use less than half of the square footage per employee

Relative to what? Here are some portable room dividers: https://www.screenflex.com/

You would be starting with a very tiny space if that doubles the square footage per employee. They completely eliminate visual distractions, give employees a feeling of privacy, and stop a lot of noise. Being portable, they can be changed around as needed, and you can easily create small groups.