The fastest way to obtain a Bulbasaur in any of the Switch titles would simply be transferring in the Hidden Ability Bulbasaur you get when starting Pokémon Home into a save for Sword/Shield, Brilliant Diamond/Shining Pearl, and Scarlet/Violet. Though, this Bulbasaur will be disobedient if over-leveled. This isn't an issue for Scarlet/Violet however, as obedience is based entirely on met level in those games instead.
If you're interested in playing a game where Bulbasaur is naturally obtainable at an early point, then Let's Go Pikachu/Eevee is your best bet. You can encounter Bulbasaur as a rare spawn in Viridian Forest before the first gym or, if you'd rather skip the trouble of catching it, receive it as a gift Pokémon in Cerulean City.
If you have the Expansion Pass DLC in Sword/Shield, you can also get a Bulbasaur capable of Gigantamaxing after clearing Mustard's first trial. The Isle of Armor is accessible fairly early into Sword/Shield, so you can likely get this Bulbasaur before playing through most of the game.