are you thinking Hydroban to waterproof the plywood? yes. Is there a way to remove this thinset and salvage the 1/2" plywood so I can leave it down? The way I do it is to use a grinder with a vacuum shroud on it. But that's pretty expensive to buy, and very few rental places will have it (although you could ask). That would take it right down to clean plywood. In the absence of a shrouded grinder, I'd replace the plywood.

"How do I pick a tile that isn't a trend and go out in a few years and one that I'll like for a very long time. " Everything is a "trend" sooner or later ... the trick is to pick something that you really like, preferably something you have always liked. Simple has longer staying power than fancy or "cute" Neutrals and classic "bath" colors (like cobalt or pale blues) last longer than trendy ones Pick something you can tart up with trendy accessories, that will look good if this season's magenta towels are replaced in a few years with yellow.

I'd go with the 18 X 18. My master bath has 12 X 24 on the shower walls, 12 X 12 on the shower floor, and 24 X 24 for the non-shower floor. It doesn't look busy because it is a very uniform, low-key pattern. If you use a linear drain, you can use any size tile since the shower floor only needs to be a single, flat plane. I dealt with the issue of slipperiness by getting a tile that has great skid resistance. They aren't rough, but they have an almost unnoticeable striation on the surface that makes them entirely safe. I have another shower where the tile is very slippery so I went with 2 X 2 tiles: this means I have a lot of grout lines that increase the traction.

Agree, just the shower area. These are nice tiles for a shower :)