pigment pens are kinda like sharpies except they deliver a line of actual oil based paint (I dont know what they use for the diluent).
Ive used them to sign paintings many times. You can get em in many colors and in metallic too.
They wont last years , but if stored properly(point down so it seals the tip) itll make it a year or two.
I assume you wont use it a lot.
IF YOU USE A SHARPIE--DO NOT spray it over with fixative or varnish, the Sharpie will become "paper chromatography"
(SINCE WE LAST TALKED) I tried to write on a canvas back and, while it sunk through an unpainted canvas, it didnt seep through a gessos and painted one (but write on the wood if you don't wanna try it firts)
Qlso, the writing "OVER" your signature on the front with a sharpie is not a great idea because if you varnish it as I said, the Sharpie will seep and run like a chromatograph