Apple

Dislike my ecobee pro in my current house. Liked my first gen ecobee in my last house. The new one has a poor quality air quality sensor and I’ve had to calibrate the temp for it to feel proper. Also had to replace a unit that lost its connection with HomeKit and wouldn’t rejoin the home.

For me, my next solution will be based on what works best in Home Assistant.

To be fair, Ecobee isn’t the worst thing in the world, just not a brand I’d buy from a third time.

I’ve installed multiple smart thermostats over the years. I find that Eccobee is the new stand out (for that price). It bests nest in many categories and offers all the bells and whistles you mentioned (including multiple room sensors). It has some features that i find unnecessary (Siri, etc), but you may find those useful.

As for smart smoke detectors…i still think nest has the best and easiest option. It doesn’t play nicely with Apple HomeKit —— but it can. I’ve tried HOOBS and various other options to support Nest, but none have been easy, intuitive, or worked particularly well. Then i bit the bullet and tried the Starling Home Hub. It’s about $100 and works flawlessly with all nest products (even the old nest thermostats). It’s easy to setup and one of the most reliable HomeKit hubs I’ve used. Truly “set it and forget it.” It’s given new life to first gen nest cams, a thermostat from 2013, and various older smoke detectors.

If i were in your shoes I’d add that first and see how well it performs with your existing nest hardware (assuming it is still working). The Eccobee is a nice upgrade if needed and can easily operate alongside starling/nest smoke detectors.

So you already have HomePods? Because if so, all of your cheap dumb smoke alarms automatically become smart. It’s just recognizes the alarm sound and alerts you.

+1 for ecobee

For smoke alarms, I recently picked up one of these Z-Wave smoke alarm relays. It attaches to your existing smoke alarm interconnect circuit, and pairs with a Z-Wave hub such as this one. Add that to Home Assistant with Z-Wave JS, and pair it to HomeKit with the HomeKit Bridge integration, and you’ve got smart smoke alarms!

I use tado and am very happy

Not the answer you were looking for, but these guys sell a solution to get Nest stuff working in HomeKit: https://www.starlinghome.io/

It’s annoying you have to pay another hundred bucks to get this integration working, but it’s an option.

I’ve been using my Tado thermostat for the past 5 years without any issue. It integrates well with HomeKit and home assistant. Its radiator valves are great at separating the heat around my home.

For smoke / CO detectors, I just have dumb ones. But I’ve noticed my HomePod mini has urgent notifications when they sound. Don’t know how accurate these are as they require sound recognition.

Nest with Matter. Depends on the functionality you're looking for, but I'm super happy with the newer Nest Thermostat in HomeKit using the Matter support that was added earlier this year.

It doesn't support eco mode or learning functions, but it does everything a traditional thermostat can do from the wall, but from HomeKit. Plus I can script temp changes and min/maxes using Automations.

Nest has also had some huge discounts, I got two through my energy provider for $40ea. Sure beats a $200 ecobee.

Ecobee works, but I’m not a fan of dealing with the thermostat’s built-in UI. It also doesn’t have an “eco” range setting like Nest does. If you’re inputting everything through the home app, you’ll probably be fine. Ecobee also sells occupancy+temperature sensors that you can use to tune the thermostat further.

I like my Ecobee a lot but the one thing I’d look for next time is something that does not require the vendor’s cloud service. Should be able to have full local control because eventually all cloud services shut down.