>Introduce a 19th type called Null.
>The only pokémon of this type are Unown, Arceus and Silvally.
>Null type functions as the type of the user's top-left move at all times, or Normal by default if it's undefined somehow.
>It's not a huge change to the type mechanic, because several existing types already grant immunities to status effects, therefore types can be more than just damage modifiers.
>This way, Unown is necessarily the same type as its Hidden Power.
>Arceus and Silvally still have their abilities to change type based on held item, but now they can be dual-type if the move's type and item's type are different.
I see no reason besides laziness why this couldn't be done. You may not find it competitively viable, but it fits the lore better.
From a mechanics persepctive, an ability that does nothing but replace the user's type effectively means Arceus and Silvally get no benefit from their abilities during a battle, because all known means to switch a held item in battle are very impractical.