1) the whole mattress solution
OK. If you want to replace the whole mattress, Eight (originally called Luna) is popular, can set up heating zones, works with Alexa or Google Home or IFTTT (2nd generation or newer model). SmartThings Integration would probably be through IFTTT.
Their first generation models just weren’t very comfortable to sleep on, but the second generation have much better reviews.
https://site.eightsleep.com/pages/integrations
2) the (literal) vaporware solution
BedJet V3 is a cooling/heating system which sounds weird but seems to work. They’ve been promising IFTTT integration for awhile, but it’s not here yet. I would wait until they actually deliver.
3) the hacky IFTTT robot finger solution
One other option, but I emphasize again that for fire safety reasons this should only be used with a full mattress pad that stays fixed in place, not a heating pad or blanked that might be folded over, is to get one with a controller that has buttons that can be pressed by a “robot finger” like Switchbot. There are fewer of these than you might think because the Switchbot can’t do capacitive touchscreens and the button placement has to be far enough apart. Sugru will help fix the Switchbot at the right angle.
The only one I’ve seen that works pretty well and has good safety certifications is the Serra Sherpa mattress pad in this specific model, 860938 . The nice thing is on/off is a toggle, so you only need one Switchbot plus the Switchbot mini hub.
https://www.amazon.com/SwitchBot-app-controlled-device-mechanically-Midnight/dp/B07B4D9KVX/
https://www.amazon.com/SwitchBot-Hub-Mini-Smart-Remote/dp/B07TTH5TMW/
https://www.amazon.com/Sugru-Mouldable-Glue-Original-Formula/dp/B008MIRQUE
And here’s my recent review of the Switchbot:
So I know that’s a lot of different pieces for the project, and a fairly high cost ($175+$40+$40+$10=$265) and if you want to be able to adjust the heat rather than just on/off you’d be adding another 2 switchbots, but it should be both doable and relatively safe.
I don’t know whether to expect more devices in this category in the future, the fire safety issues make it a high risk item for manufacturers and a lot will just pass.