If you just use the Ifttt service, it has a free tier that includes the weather underground information. You can only set up a couple of rules that way, but it may be enough. They also have a paid tier, which let you do more. There’s a cloud to cloud integration between Ifttt and Smartthings. So you use the Ifttt weather service as the “if“ and the Z wave switch as the “that“ in a rule that you create in the Ifttt service. Ifttt sends the request to smartthings to turn on the switch when needed.
I’m not familiar with what’s available through the weather feature in the smartthings app and it does vary by location.
As far as using a mains powered device in the garage and then running something outside, some people do that. There are some devices that can attach a temperature probe. Shelly is popular for this because it’s a Wi-Fi device, so you get really good range. But there are also some Z wave devices that accept a probe.
The following recent thread lists a couple of options that are known to work with the 2023 smartthings architecture. I’m sure there are more, I just don’t know which specific ones. Hopefully other people will post with more suggestions.
Devices that work with DS18B20 Probe sensors that have working Edge Drivers (2023)