I need more info. As for the grout line size thta is what ever the tile maker suggests . IMO black grout is always hard to keep black and in a shower ever worse . What tile are you using? Where are pics of the waterproofing in the shower ? I think you have too many finishes in he space . Why 2 very different faucet and shower head fixtures style wise. I never like rugs in a bathroom only a bath mat that can easily be washed .IMO the watwrproofing should have already been done before the tub was installed.The flooring ahould have been installed before toilet and vanity were installed.