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.