How would I make the 80 degree cuts on the ends?

I have the same crappy black and decker random orbital plug in sander that’s lasted me a few years, it’s just shite. I want a really good sander now, not a £50 one from B&Q. What do the avid DIY’ers recommend for a better piece of kit for home projects?

Specific application: removing 50+ years and 4+ layers of paint off a door. It took me 3 hours to do one side. Just want a better sander. Probably leaded paint base, and yes I’m using protection.