I don't know how long ago you tired this (early versions were, even I will admit, kind of crap) but in the current FreeCAD release of 0.22.1 it's incredibly easy.
Just select the edge(s) you want to fillet and and press the "fillet" button in the Part Design workbench...
Or, if you want to get fancy and make your own bespoke custom fillet. Well, you can define its profile in a sketch. Here I just used a bezier to make an arbitrary curved shape. You are defining the profile of the material you want removed here, i.e. the negative space. You can make this as mathematically rigorous and precise as you like.
Then, position it somewhere on the vertex you want to profile and once again in the Part Design workbench, select that sketch, and hit "subtractive pipe."