Darn, the rest of my comment went away when I went to look for pictures on Houzz. Okay, you can still use the same size shower pan if you do either of these two ideas.
1. Put in a bench on the left side of the shower. The bottom front wall of the bench goes into the flange of your shower pan, just like your pony wall does. But that wall goes up only 18” and then becomes a bench, whose top goes all the way back to the outer wall that is stuffed with insulation. Use the same material for countertop, curb top, and bench top. This widens your shower without changing your plans for the size of the shower floor.
2. Start your left side wall just like you have for your pony wall, but then insert a ledge about 10” from the shower floor. Ledge has a slanted floor and is for shaving legs.
Then continue your wall up until you get to a good height for a niche. But instead of a niche, put in a full length ledge. The left side wall of your shower will now end almost all the way to the wall with the insulation in it. Stuff that small extra space full of more insulation to keep from having heat loss (or gain, depending on your season and climate) because your left shower wall is an outer wall of your house. Instead of drywall on the front of that small wall, use the cement board and redguard or whatever waterproof material the rest of the shower is built of, just to be sure you never have problems from condensation. Use the same material for the bottoms in the tilted shaving ledge and the slightly tilted ”niche” ledge as you use on the curb and the vanity counter.
Or, put a wall in front of the bench, and do both a bench and a ledge.
Q
Joan
Q