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This installed fine, but gave me an error of an out-of-date npm version, so I ran: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!

Seems my reply to this got lost…

Got this working! Removed the .nvm directory, removed the symlinks, shutdown pm2 and re-ran the installation script and it all went in fine!

sudo rm -rf ~/.nvm
sudo rm -f /usr/local/bin/node
sudo rm -f /usr/local/bin/nvm
pm2 stop mm
sudo rm -rf ~/MagicMirror
bash -c "$(curl -sL https://raw.githubusercontent.com/MichMich/MagicMirror/master/installers/raspberry.sh)"

I got an error on npm not being the right version, so upgraded that:

sudo npm i -g npm

(Note sudo is working now, which it wasn’t before) And now my node and npm versions look good and pm2 is running MagicMirror again!

pi@raspberrypi:/ $ node -v
v9.11.2
pi@raspberrypi:/ $ npm -v
6.5.0

Thanks everyone for the help!