When I did my in floor heating in the master bathroom I went through something similar regarding the zones.

My goal was to be able to control a zone for the shower and a zone for the floor tiles everywhere else independently of one another.

Two separate lines are run in these areas under the floor to one two zone thermostat, on a dedicated line to my breaker panel. From what I found out during my process is that a two zone thermostat controls both zones at the same time.

I could not find any thermostat control panel that controls two lines independently and figured if I ever do a whole home in floor heating system each room would need its own thermostat.

Plus side to a two zone thermostat in a room is if one of the cables is cut in a remodel the other zone still functions.

ERVs should be controlled by humidity, outside temperature, and time. Some come with an automated mode that adjusts run time and fan power with those. It'd probably be difficult to find controls that have multi zone, HA integration, and E/HRV features in one package.

I'd leave the ERV controls out of this one.

I haven't used Homekit, so I can't speak to that, but I have had an ecobee for several years now, and it's integrated into HA.

It's not clear to me if your setup is "1 heating system for one room, and a second system for a different room", or if it's two heating systems for the same area.

In the first case, I don't think it'll work - ecobee is only designed to control one heating system. The remote thermometers just tell it what's going on where they are, so it knows if that room needs more heating or cooling. If they do, then it fires up the HVAC to make that happen. You can tell ecobee which thermometer(s) to use to trigger the HVAC - at night, you probably care about the bedroom more than anything else, while in the day you probably care about the living room more. But it can't simultaneously control two separate systems.

If it's the second case, ecobee does, I think, have provisions for two stages of heat, so it's possible you could set it up to use the second HVAC as stage 2. But I'm not certain about this at all. You might need an additional relay, I'm not sure.

But I think there might be an easier solution that doesn't require a full blown second thermostat using HA, and this is what I do for our pellet stove (we have a main household HVAC, and a pellet stove for extra heat). Get something like a Shelly 1 and use that to trigger the second heat system. Then put in a temperature sensor - an ecobee remote sensor, bluetooth, wifi, Zigbee, etc., whatever works best for you - and with those two things, you can create a "thermostat" entity in HA, then use a thermostat card to control it. Assuming HA is running and it's getting info from the thermometer, it works well. Of course you'd mostly access it on a phone or computer, not something mounted on the wall (unless you put a tablet on the wall to view and control HA, but that's a whole different project).

This isn't necessary for it to work, but I have a script set up that has logic such as: []If the outside temperature is above 50 degrees, set the thermostat to 50 so the pellet stove shuts down and doesn't restart. (Don't need it.) []If the outside temperature is below 45 degrees, set the thermostat according to the rules below. []At night set it to 70 degrees, during the day, 72.
[]If the outside temperature is below 32 degrees, set the thermostat to 78 degrees so the pellet stove stays running.

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.