Open-plan is not an option.

But focussing on how to grow might help. If I stick to the max 30 people rule, I will soon learn that I can do a lot with 30 people.

And if not, why do they have to be so closely integrated? Communication gets hard anyway in such a large group. Why not grow to 40 and then split into 20+20 and make the two teams two separate businesses, with their own office and at the same time reducing the width of the communication channel between the teams.