Resource: archival-quality pens, not fountain
if you like fiber-tip-like finepoint pens rather than fountain pens or rollerballs, and don’t mind using another disposable stick, you might very much like the Sakura Pigma Micron Pen M901 (pH neutral black pigment ink). These pens (which come in several different colors) are much-used by professional conservationists, archivists, photographers — and now you, to write your ethical will.
These pens are readily available at art supply and photography stores, and online.
So far we have not found a ballpoint pen ink that is pH neutral/acid free/archival quality but if and when we do, we will surely post it. More archival materials coming soon.