On the equipment end, use separate power amps. Use 1:1 ratio for amp channels to speakers (rooms). So 30 rooms = 60 channels of amplification. Start with less than 30 sonos connects. Maybe 15-20. Gang groups of rooms together where it makes sense. Brings the cost of entry down, and if the client doesn’t like it, you can zone it out more down the road with more Connects.

Another approach is to use passive volume controls (for impedance matching) and fewer amp channels. So single source, multiple volume control for groups of rooms. Very cost effective but plenty of drawbacks.