Vent: My neighbor in Phoenix says a backyard studio needs a full bathroom, but I built mine three years ago with just a sink. | Backyard Studios - The Design Community Hub
Honestly, saving that $8k sounds like the right call for how you use it. Tbh if it's just a workspace, a full bathroom is overkill. My cousin did the same thing and never had a single buyer complain.
Your neighbor has a point about resale, but you're right too. Three years without needing one proves it's just nice to have. Saving that cash was smart, especially if you just use it as a workspace.