Aha-got it!
sudo rm -rf ~/.nvm
sudo rm /usr/local/bin/node
sudo rm /usr/local/bin/nvm
pm2 stop mm (not sure if I missed this earlier)
bash -c "$(curl -sL https://raw.githubusercontent.com/MichMich/MagicMirror/master/installers/raspberry.sh)"
This installed fine, but gave me an error of an out-of-date npm version, so I ran:
sudo npm i -g npm
Then moved all my old modules and config file over to the new install and checked my node/npm versions:
pi@raspberrypi:~/MagicMirrorBackup/modules $ node -v
v9.11.2
pi@raspberrypi:~/MagicMirrorBackup/modules $ npm -v
6.5.0
So they’re not as up-to-date as my previous ones, but MagicMirror is working fine!
Thanks everyone for the input, turns out just a bit of manual removal and persistence sorted me out!