Would you put a smart lock on your window or is that just getting weird?
The battery-powered contact sensor is probably the better middle ground, but I’d check one detail before choosing it: what happens when the internet is down?
Some sensors report the open/close event to the hub locally and then sync it once the connection returns. Others depend on the cloud for everything, so an internet outage can mean no alert and, in some cases, no useful event history. That’s different from a Wi-Fi outage too, since a sensor using a separate local protocol may still reach its hub.
I’d test it before mounting several: disconnect the internet, open the window, wait a few minutes, reconnect, and see whether the hub records the event with the correct time. It won’t provide a real-time warning while the connection is down, but at least you’ll know the window was opened. For an old frame that shifts with the weather, I’d still treat it as monitoring rather than a substitute for a mechanical lock.