that’s a known issue with no water-proof fix. co-planar faces with baked lighting do produce those differences. you can try to minimize the effect with various solutions like better lightmaps, higher lightmap res, tweaked lightmass settings, etc. but your best bet would be to design your modular pieces in such a way that you either do not have connecting edges on flat surfaces or add some architectural feature there to hide the connecting edges.
First is your lightmap UV’s, they are split up too much, you have some very small islands there which need to be attached to other pieces otherwise it requires a high resolution lightmap to avoid issues.