Biggest secret of getting level tile is making sure the subfloor is dead level first.
This will be for the shower walls, not the floor. The floor will be done with (unsure the correct term for it) those flexible mats of 12"x12" small connected tiles (like this:
https://www.homedepot.com/p/TILE-CONNECTION-Sliced-Pebble-Tile-Grey-Black-White-11-1-4-in-x-11-1-4-in-x-9-5mm-Honed-Pebble-Mosaic-Tile-9-61-sq-ft-case-xs3pabk/308170074), because the floor has a slope on all sides to the drain in the middle of the floor. The fiberglass backer mat will make adapting to the slope to the drain fairly easy, I hope. Leveling should be unnecessary for irregularly shaped sliced pebble tile like this, other than perhaps compressing each section into the thinset by using a piece of plywood to even the pressure out across the section.
I've completely removed the previous greenboard backing material on the walls and will be doing cementboard backer on all the walls. assuming my studs have not warped, levelness should not be an issue. I did have studs near the door suffer from water intrusion but I've cut those out and replaced with new lumber.
I've also read that mastic is bad to use for showers. It's organic and not water resistant, and can contribute to mold growth later on if grout ever begins to fail. I'm planning on using thinset.