Think you’ll find your shooting through the memory on the pi the only one that I’ve managed to have running for two days is the pi 4 with 4Gb of memory that seem to be ok on memory, but needs a big heatsink and an active fan, temps on min have been 61c but that’s not with the fan active my fan only activates when the tv is on then again the nest has recorded temps of 38/39c in the house so it’s not doing bad, the Pi4 is the hottest of the bunch.
You say it’s not running it the screen off or the pi just running raspbian normally pm2 is running all the time so regardless of it being on a different screen or the screen turned off MagicMirror is running, you would have to pm2 stop MagicMirror in terminal but 47c looks like normal temps