The only known way to convert 100% of matter into photons, is a matter-antimatter annihilation. You are bound to encounter some antimatter over enough light years of travel, but it isn’t clear whether it would be enough to annihilate all your matter, and the ship’s matter (there doesn’t seem to be too much antimatter out there).

At some much earlier point though, you’re going to receive such an amount of high energy radiation, that the whole ship and its occupants, are going to turn into a ball of plasma… including the engines, so no more accelerating from there on.

That ball of plasma is going to collide with interstellar dust at quite high speed/energy levels, just like in a collider, with the particles breaking apart and creating a cascade, of photons and other particles, that will quickly decay and/or coalesce into other ones.

So you will become a photon, even a lot of photons, and while some would escape in random directions, the plasma cloud would dissipate and slow down over some distance, becoming mostly interstellar dust itself.