grab indeed pays better, only question is will you still have energy to study after such a physically demanding job under the sun. Also there's no transferable skills and no career progression doing grab.
Sales engineering is pretty decent if that's what you enjoy, even at 2kish salary, you're actually getting bout 2.5k after cpf. If you're desperate for cash, can also do food delivery on weekends during peak hours where you have no classes. Lastly, another option to consider is signing on and suckering SAF to sponsor your degree if your diploma is in an engineering field.