Hello Melissa, welcome to Permise!!!
Cobb is a wonderful form to pick...and many examples around the globe to choose from.
Obviously, most cob houses have curved walls...
Actually, most do not have curved walls, most actually have timber frames of some form that form the wall infrastructures and the cobb (or... Clay- Slip Clay Straw/Chip, Clom-Tabya-Adobe-Bousillage-Colombage-土壁 (Doheki), 土塀 (Dobei)-Bajareque...etc) is only the infill portion of the wall. Some are organic in shape like you find in Africa, and some of the "earthy and new age" ones folks are building...yet the vernacular is straight walled in most examples.
You can build an earthen tub, but this is not easy or for the "novice" builder. It will also require a lime finish of
Tadelakt or similar natural water resistant plaster as seen in your shared photo. You absolutely do not want to use OPC (concrete) on any natural building...especially earth matrix materials. I too, would recommend a salvaged claw foot, or
wood tub, though cobb one is possible. Are you drawing the blueprints by hand or are you using a program like SketchUp?
Good luck, and again welcome!