Can you harvest any times from hidden areas such as a closet?

You get close then you glaze the new tiles (or hire a faux painter to do it). Yes it will wear off of high spots but will wear to a similar color.

Contact Mission Tile West in Pasadena, CA. They specialize in this type of product for the many older Spanish style houses in Los Angeles.

I'm actually looking for a more artistic approach. I am confident I will not be able to match size, shape, and color on these tiles; so why not go bold and different? Just as an example, using unglazed saltillo and dying/tinting them. Or an entirely different tile.

Talk to MIssion Tile West, as Chispa says. They have amazing products and expertise.

https://www.missiontilewest.com/

I would talk to these people in Austin, TX

Link: Rustico Tile

Sorry, but using wall tiles on a floor, like that turquoise, is not a good way to make what you have work. Not at all. It would look haphazard and unplanned, and just generally bad. Because it was haphazard and unplanned and generally bad to start the project without finding replacement tiles to look seamless.

Don’t try to match Saltillo. Won’t work.

Best move: use a different material in the gaps — stone, brick, or decorative tile — and frame it clean so it looks intentional. Like an inlay, not a patch.

For big spots (like the jacuzzi): turn it into a feature. Herringbone, patterned tile, or even sealed concrete.

Keep it simple, clean, and deliberate. That’s how you make it work without looking like a mistake.