Thank you! It was definitely hard to stick to just two types, but what led to not choosing Steel here was it’d still be possible for him to learn Steel-type moves without being a Steel-type himself. Perhaps his typing could also differ depending on his current Alter/form.

While Jirachi is the only Mythical Steel/Psychic-type Pokémon, the Beldum and Bronzor lines, as well as Gen 9's Iron Crown, share this powerful combo.

Solgaleo of Pokémon Sun sports the inverse (Psychic/Steel), which carries over to Dusk Mane Necrozma in Pokémon Ultra Sun.