Is frequent sleep-wakeup harmful to the electronics?
The hardware itself won't care, but if you're using an external crystal then it will take some time before the clock stabilizes, on the order of tens of milliseconds. If you want quick wakeup then you should switch to using an external oscillator (and possibly clock distribution ICs, if you want to share an oscillator between several to several dozen MCUs) and then configure the MCU to skip as much of the clock stabilization delay as possible.