The smallest thickness of Tantalum sheet is 0.025mm , Normally we call it Tantalum foil with the weight 1kg or 2kg per roll ,
The thickness less than 0.1mm , can be made into foil ,
The thickness more than 0.1mm , can be made into straight sheet ,

controlling annealing temperature (>=2 times normally)
Using different annealing method.
Put in level, using heavy block above them in the vacuum furnace, to ensure the better flatness.
Put in circle, without anything above them, the surface is shinny and perfect, without any stain.
Put just 1pc for annealing, cost is high) if you have enough money
choose good carbide blade, do polishing, grinding, use CNC machine, etc