Or, if you want to continue the herringbone look, the Color Wheel mosaics has a matte black herringbone for the entire floor. It's DCOF rated for showers. https://www.daltile.com/products/Wall/Color-Wheel-Mosaic/Matte-Black

Before installing black tile on a bathroom floor, think about this...

Grout between even a 1 x4 mosaic will be enough "grit" to be non slip/

Your biggest concern, despite the drama? Darkness. You need to light the daylights out of that room. Seventy six is not OLD!! but eyes tend to go first : )

Clearances matter, and the sweep on the floor of a curbless shower with a door any size, will send water out to the bath floor. That used to be a tub /shower combo? Then it is narrow! as my guess is this was formerly a 5" x 9" ish bath, at most?

Safe clearances are as important as the tile. I'll hope the vessel sink is just an inspo: ) as well.

This is a new bathroom—though half of it was a smaller old bathroom—with a shower-only. And yes—the vessel sink is not part of our plan. I will pay attention to the splash area and door sweep water-spread for safety. Thx.

Does anyone know if there is a material to add on to tile to make it less slippery?

This article might help https://www.thespruce.com/coefficient-of-friction-specs-to-buy-best-1822608