Well, after a bit of research, I found that Sleep as Android, which seems to be one of the top sleep tracking apps, may be able to do what I'm looking for. The idea seems to be that it vibrates or sounds when it calculates that you are in REM, but not enough to wake you up. It seems like it's compatible with the Samsung Smartwatch, and some other wearables. I'm finding setting it up all a bit daunting, to be honest, but I'll set aside an hour or two to work it out soon. I'm still working on recall more than lucidity, so I'm not in a rush. Here's a line to the app documentation:

https://docs.sleep.urbandroid.org/sl..._dreaming.html