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Can’t tell if that would look good or shit
Imo you need less rustic/more modern boards for that to work.
I think what's on there kinda works with the tiles. So I'd be tempted to see if you could get a single sheet of ply cut to size and then paint it a dark grey / light charcoal with a bit of a greeny tone to tie into the tiles. Something like F&B Rock Bottom.
Main challenge will be finding something big enough for that 4m long bath.
I’d be tempted to run the boards vertically up the bath
Can’t tell if that would look good or shit