Measure the wall and determine where the cuts will fall on the different tile sizes. That will help you decide what will accept the cuts better. Note that if your walls are not straight, the corner cuts will emphasize any wonky walls.

IMO the stacked 3x6 won't install well with the hex tile.

Maybe consider a 1x1 square tile for accent.

Here are examples of showers with accent tile walls. Maybe this will supply some inspiration. Note these are 1x1 mosaic accent tiles

Decide if the accent wall should be the wall other than the wall with the plumbing. It makes more of a statement.

Here's another way to use subway tile & accent tile

Thank you so much for the detailed reply! This is very helpful