But we're talking three decades -- 30 years -- worth of buildup here. This toilet was new when Apple first released Macintosh computers. John Lennon was still alive. No one had even dreamed of AIDS yet.
I don't mean to be rude, but we aren't talking about the buildup on the little holes in your showerhead, here.
In using the acid, I would use the same caution I use in the chemistry lab and also when I make soap, which I do using highly caustic lye: heavy rubber gloves, safety glasses, large shirt (no lab coat at home) for catching spatters, neutralizing solution within reach, windows open and fan on.