Thank you, PJ! “LowPower.deepSleep(500)” works plenty good enough for my requirements. During sleep, total system draw on battery is under 0.4mA, and about half of that is loss from the step-up to 3.3v converter (85% efficient). The surprising and cool thing is that Serial1 works fine across sleeps with no detach/begin after setup.
Originally I figured sleep would be woken by external interrupt, but that’s difficult and unnecessary. When it wakes, Xiao can check whatever it needs to see if it has a reason not to sleep, i.e. user is adjusting a pot knob.
Thank you!