Rotation will wipe out all competitive demand for these cards, but cards of Pokmon like Gengar will be largely unaffected since the vast majority of their demand comes from collectors
Thankfully, you might already have Evolved Sleeper if you play Arena since it sees play in various tier black decks, so hopefully you won't have to spend wildcards to get it
After all, the variance in rarity is deliberate in terms of the distribution of cards in booster packs
The Legendary Diving Pokemon was the subject of Pokemon The Movie 2000, a film that captured the interest of kids during what was likely the peak of the Pokemon zeitgeist
Access to all 5 colors makes this considerably easier, however, and effects that give all your creatures haste like Fervor are able to really shine when youre setting Garth up for combos, as you can make infinite Shivan Dragons with a little preparation
Aaron Miller did a great job infusing those colors into the piece while leaving his costuming the traditional grey that is so iconic for Gandalf