To keep the cube from turning into a sphere you adjust the subdivision setting here. Set it to 0. Then press the subdivide button.
That option is also found here if you are using the new layouts (which I personally do not like).
What do your UVs look like? From your bake it does not look like the UVs are an unwrapped box at all. The base mesh, ie the cube, should be UV mapped.
Your UVs should look like this, making sure to add a bit of spacing around the edges as well.
Typically you would never bake a final sculpt down to a standard cube object. So your test case is a bit extreme when it comes to baking. Instead you would sculpt your object to create a base mesh, then retopologize that model and continue sculpting from there. Or you could project your high res sculpted detail onto your newly created low rez version, clean it up if required and then bake the high detail down onto your low rez version. That way the low rez version has UVs that are created in a way so that any seams are not showing when rendered.
If you are a Maxon One subscriber you also have access to Z-Brush.
kbar's post in Looking for a freelance Cinema 4d plugin developer was marked as the answer