You can certainly do a brick or stacked pattern in the shower and a larger format tile for the floor. This is not overthinking AT ALL. This is simple design 101, in fact, the same stone in different sizes and even shapes (like hex) is very inviting to the eye and it breaks up the monotony. Also, there is nothing wrong with different colors for grout on floor and walls. It sounds like a 12x24 might be a bit big for a herringbone pattern unless you have a LARGE floor to lay it out on. Even so, the diagonal look is very interesting in a 30cm tile for example. Your original idea sounds fine!!!

Both the extremes in the 'sheen' family can be deathly difficult to live with. Polished (or gloss) shows dust and dirt and paw prints because it is SOOOOOO shiny that any flaw shows itself. A MATTE finished floor shows oily paw prints because oils (skin oils) are HIGHER gloss then the matte. Therefore you SEE them. Matte is the LOWEST on the sheen chart (extreme low) and Polished (glossy) is the highest on the sheen chart (extreme shine). Both of these are DIVAS and like any Diva it should come with a live-in maid. Personally I would be more concerned about the SLIP rating. Someone with wet shoes (or any person wearing high-heals) is going to be slip-sliding into the walls, through doorways and eventually 'arse-over-tea-kettle'. I always question the decision to install GLOSSY tiles in malls. It is inevitably a man who made the decision. Any woman who has worn a narrow heel on a slick/shiny tile will tell you NEVER use them as flooring. I guess that tells you how many women have a hand in designing shopping malls.

I am not sure polishing has anything to do with it but any smooth surface flooring is slippery especially when wet. I had vinyl tiles in a bathroom (12" x 12") and they were a death hazard. I rented a place with marble 12" x 12" marble tiles and they also were a hazard with the least bit of moisture on them. Consider that most commercial buildings and apartment buildings with smooth surfaced lobbies put out special little yellow warning things when it is rainy, snowy or even when they have finished mopping. There is a COF rating for tiles so you should start there. You could also get a few sample tiles and see what it is like to skid around on them.

porcelain tiles, matte/shiny, all clean the same. not a bit deal. tiles on the shower floor (mosaics) can go up to 4". the grout provides traction. you can do larger tiles on the floor if they are suitable for wet areas and are a matte. but using larger tiles on the floor is difficult w/a slope. ifi you have a great tile guy, you can try something like this, (it's a honed basalt) but using a mosiac is easier. always look at the specs on the tile you're considering. it will plainly tell you about usage (wet or dry) , cleaning, and how to install mosaics are fine w/kids. look back into the bathrooms from 100 years ago. they always had mosaics. the grout provides all the traction. using a larger honed or matte tile is also fine. I have a 4x16 marble (honed) in a chevron layout in my bathroom and it's not the least bit slippery In my shower I did a 3" marble hex tile. also not slippery floor I did for a client. 12x24 charcoal porcelain outside, porxelain mosaic in the shower you could also go w/a larger, textured type of tile if you don't want a mosaic on the outside. Ivy Hill Artmore Venecia is a good one. it's porcelain that looks like a terrazzo. it has some 'bite' to it so it's not slippery.