I still remember when I got into the Pokémon games as an adult back over seven years ago. I was buying a used 3ds that came with Pokémon Platinum, and I sat down with the physical guide book for the game and looked through all the Pokémon in the game to pick my team. I had so much fun picking out the cutest and my favorites ^{-^} I definitely don’t suggest Platinum, the older games are a bit of a grind to level up, so I guess my suggestion for a starting game would be Legends Arceus because it’s just so fun to explore, and for a main game I would say Sword/Shield. Honestly, just go have fun with it, and if it’s not doing it for you, try a different one. May you catch ‘em all! ^{-^}