Something to do:

Reduce the Shadow Accuracy to the same as the Object Accurary (I always use 0.5). This will make shadow more shapen.

Turn off the Interpolation in the Global Illimination tab (you are using a 4 pixels shadow pus interpolation... this will create big artifacts in the final render)

Use Lightning Quality to Fast and 1 pixel to get better results (I know the render will slowdown but you will end with a more quality)

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