I don't think it's the timers. I have disabled them (though maybe not all / they've been reset by some update).

And this also happens during the day, say if I put it to sleep while going for a walk. It sometimes wakes up a few moments later, sometimes it'll sleep all the way.

My schedule is fairly regular, too, so unless the timers have large jitter, I'd expect them to trigger roughly at the same time (either after X time, or at Y hour).

This used to happen on Linux initially, too, and there were no timers there. I could stop it by disabling EC wake up. I've never found anything similar on Windows, so I just turn it off. At least it boots quickly...

It might not be useful after the timer goes off but you could try looking for them via

   powercfg /waketimers

Or checking what last woke you via

    powercfg /lastwake