No, I would build my mesh, set the whole mesh to “Shade Smooth”, set “Auto Smooth” to an angle of 180° and set all edges I want to be hard (no smoothing) to “Mark Sharp”. UV layout is my very last step.
I am not familiar with the term smoothing groups because Blender has none. In the image below I set the object to Shade Smooth and marked some edges as sharp (indicated in blue color).
In Blender the object looks correct with it’s smooth and hard edges.
Exported as FBX, in Painter it looks correct too. Hard edges seem to be carried over. I haven’t tried to bake things, would have to set up a high poly first.
But what I have tried with great success for this (admittedly) simple object is to bake in Blender a normal map with just this one low poly mesh and in the material setup using the Bevel Node (not modifier), pluged into the normal connector of the shader. This gives a normal map where the hard edges turned into, more or less, subtle rounded edges.