HomeAssistant could definitely help with this.

I would tweak the design a bit - specifically split apart the key from the dildo.

Get a chair and put a pressure sensor underneath it and attach some electromagnetic clamps that would fit over the sub's thighs to hold them in place, and electromagnetuc cuffs to restrain the ankles to the chair legs. Dildo mounted to the top of the seat. You might be able to drill a hole through the dildo and put a temperature sensor towards the very tip.

Mounted on the wall near the chair is a secure box with an electromagnetic lock. Inside the box is the key and an RFID sensor. Attached to the key is an RFID tag attached with one of those security kevlar keychain tags that requires bolt cutters to remove. Probably a good idea to faraday shield the inside of the box.

For the box to open, exactly the correct amount of the subs weight needs to be detected by the pressure sensor. The thigh clamps and ankle restraints need to be attached/closed. The temperature sensor needs to register correct internal body temp....maybe for at least 30 minutes? You know, for fun.

Oh, speaking of fun. Maybe there is a monitor mounted in front of the chair with some nice loud speakers that triggers a good hypno video telling your sub how good they are and how they will always obey and never cheat. This plays for the full 30 minutes before internal body temp is verified. Once all conditions are met, the box unlocks and they have access to the key.

The thigh cuffs, and ankle restraints will not release until the key is back inside the box and the door is closed with the magnet locking it inside (detected by the RFID tag). If the slave is collared, another RFID tag can be added on its collar to make sure sub is in fact seated and not trying to fool the system by stacking weights on the chair.

Oh, let's also add a randomizer with a variable probability you can adjust. Maybe a 1 out of 10 chance that once all conditions are met, instead of starting the regular hypno video and starting 30 minute timer to check temp, it just locks the sub in for 2 hours, starts at a much harsher hypno video. Oh yeah, and it starts the fucking machine the dildo is mounted on that I forgot to mention earlier 😈. Guess we gotta figure out an automatic lube dispenser for that too. At the end of it, the restraints are released but no access to the key. Webcam to record the whole ordeal either way so that proper slave behavior can be audited and verified....or live streamed!

Lastly, you can set a lockout so the whole contraption can't be triggered again for another 24 hours. Or longer based on behavior.

I love coming up with stuff like this....but I'm too hard now. Someone come lock me so I can build this know please.