If you have less than five feet of clearance but still want a bathtub, don't despair. Plenty of manufacturers makes tubs in sizes small than 60". Also, a glass panel can block spray from the shower, and take up a lot less visual space than a shower curtain and rod.
If you can't eliminate grout lines entirely, then at least reduce the number and thickness. Large-format porcelain tiles come in sizes all the way up to 24 inches by 48 inches. I had a little fun with the math. If you use a single 24-inch-by-48-inch tile instead of 4-inch-by-4-inch tiles covering the same area, you eliminate 42 feet of grout lines!