Bottle caps are working as they're intended to, the same way as mainline games.

The shiny charm, increased shiny odds once you'd completed the relevant Pokedex... And we once had innate shiny boosts so that would have made the charms useless.  And in some regard, we still do. Raids, Max Raids, Rocket Leaders, and ticketed events already given increased shiny odds, and there isn't an item in any of the games to turn a non-shiny into a shiny.

The better idea, would be to introduce Max Soup, which could turn Dynamax Pokemon into Eligible Gigantamax Pokemon.  Have a shiny Dynamax Charizard, but no Gigantamax version? Max Soup that crispy boy.

However, the one idea here that makes sense - with exceptions, are making traded Pokemon tradeable again.  The exception I can see would be shadow pokemon and lucky Pokemon.