ASPost

This might be a good device to add as an "output"/alarm/waker: https://www.amazon.com/Vibration-Vibrating-Sleepers-Wireless-Hearing/dp/B07XJYRYTZ/

Good things about it:
1) It's wireless, and with a built-in rechargeable battery, which lasts a long time. (multiple nights without recharge needed, with shaking every 10 minutes or so, as confirmed during my crazy experiments)
2) Really strong shaking, effective at "force waking" a person up. (it's still not 100%, sometimes can sleep through it, but the most reliable single alarm I've found yet)
3) Very easy to have your custom Android apps control it: you literally just send one Android Intent Broadcast, and the official app communicates the command to the device. (both "turn on" and "turn off")
4) You can customize the vibration pattern! It has four different "vibration types", and 0-100% vibration strength, and you just send the intent broadcast a string of characters, with each character indicating a vibrate type. For example, I currently use "!WGGGGG;" for a simple pulse sequence, and "!WTRG0TRG0TRG0TRG;" for a heavy (and long) "force awake" pattern.
5) It's relatively affordable. ($32 atm)

I've been using it for a few months now, and I'm quite happy with it. (it's the main alarm I use)

By the way, regarding how to keep the device next to your skin so you feel it, I place it in this wristband, which has worked well: https://www.amazon.com/Phone-Armband-Sleeve-Running-Exercise/dp/B074XFRZFD

P.S. Also, regarding output devices, it's worth mentioning that there are a variety of wifi or bluetooth controlled lightbulbs, which can be controlled from third-party apps on one's phone or PC (I use LifX, but there are many other options). I think most of the "hackers" on this forum already know of these, but I figured I would mention it just in case, eg. for newcomers. I like to combine multiple "outputs"/alarms together, like flashing lightbulbs + audio playback + bed-shaker vibration.