How to remove old pancake junction box (metal brace, lathe and plaster cieling)
I'd be looking to cut around the perimeter of the box, isolating it from the ceiling and then. go to work on the steel once the vibrations from cutting it will not be transferred to the ceiling

An angle grinder will make short work of steel, but does produce lots of red-hot sparks that travel quite a distance before cooling. A hack-saw is slower, and can produce larger vibrations but will not produce sparks.

This will not be a quick job consider erecting a scaffold rather than working from a step-ladder.