A continuous weld will warp the metal, no matter how slow you go.
I've found that spot welding and filling.......is the best method.
I have seen vinyl letters layered and clearcoated,they looked like
real embossing. I believe they were cut from some extra thick material
and it was the clearcoat that rounded them at the top and bottom.
You could get a sign shop to make some letters and practice on a piece
of metal with some brush-on resin, then keep us posted.