Yeah the moment you reach for pixel level processing tricks in GIS you need to pause and rtfm.

Grade calculations are right in qgis docs and part of their lessons, and are also top Google hits.

https://docs.qgis.org/3.40/en/docs/training_manual/rasters/t...

Even better, you'll learn about DEMs vs topo maps.

For more advanced terrain analysis, see GRASS GIS (https://grass.osgeo.org/). It’s a but clunky, but accessible from QGIS via a plugin. Whitebox also has some nice terrain analysis tools (https://www.whiteboxgeo.com/).