A rather nice way to fix the temp adjust before getting in the walk-in shower.

Nothing says the adjusting valve needs to be all that close to the shower head. You can put the supply lines and valve anywhere and just run a single line to the shower head.

Here's how they plumbed the walk-in showeer in this house. The shower floor is sunken a couple inches, so if it came to it, a guy may need to fashion a rubber wedge to ease the transition just a bit. Notice where the control valve is, in relation to the shower entrance.

The shower valve:

And the back wall of the shower: