Not aware of any other residential heat alarms available.
There's a couple on bunnings site in Oz..
Do the wired Nest smoke detectors have battery backup?
My wife would spend $190 on shoes without batting an eyelid.
Women have odd priorities...!
We have a Nest indoor cam and 2 x nest protects. We have just been thru renovations so the camera was useful to see who was coming thru the house and when, setup was a breeze and haven't had it drop out once. The painters misplaced the brackets for the smoke alarms so I contacted Nest support to buy another two and they have offered to send them free of charge - great service! Purchased the protects (battery) from Amazon US and the camera from Amazon UK.