So the vanity top isn't changing? I don't know why you would put the marble tile on the walls all over. It certainly doesn't go with the vanity top.

I hope you've selected a grout color for the hex tile that matches the white color of the tile. There are too many patterns in this room.

I find the hex tile too busy with all the pattern in the room (countertop and tile veining.)

"There are too many patterns in this room"

And colors.

there will be a lot of pattern around, and the mosaic will not look like an accent around the mirror

Tile choices do not look good with vanity counter and the hex tile behind the mirror is way too busy with the wall. I would stop and retink what it is that you are doing.

You have 3 bossy elements fighting for control - mosaic (especially with contrasting grout), wall tile, countertop. Two have to go or they will always be battling. You could maybe save the mosaic if you regrouted with a matching white grout. The wall tile seems superfluous.

I would not do that large format on the wall around the door. It just doesn't look right with the hex. The colors are off a lot. Try to find a paint color than can bring all of this together. I do wish the mirror wasn't as wide so you could see more of the mosaic on the sides.

It's not horrible - it just needs an edit. The hex will calm down with a matching grout and that is what you need.

Good luck!

For 8 years I have no backsplash at all behind my vanity with zero damage or issues. Skip the tile.