OP here. I called ecobee and got great answers immediately from them. Their customer support is really good.

They said the both the Taco and Honeywell zone controllers work well but recommended the homeywell because of how its transformer is set up. Makes wiring easier.

I can have one thermostat location and have it respond to sensors setup in appropriate locations in the rooms that need it. It can also run the fan via air quality monitor (VOC) although I may end up doing this via home assistant via a dedicated co2 sensor.

Thats really good to know. I ordered Blues but wasnt aware that it could be converted. Thanks.