This answer disregards Pokemon reintroduced in the Isle of Armor and the Crown Tundra because you need to go out of your way to get them, when it's often more efficient to use one in the main region.
The fully evolved Electric types on the Galar mainland are Vikavolt, Boltund, Galvantula, Manectric, Raichu, Jolteon, Lanturn, Toxtricity, Heliolisk, Morpeko, Togedemaru, Pincurchin, Rotom, Dracozolt, and Arctozolt. I'll go over the top three, in no particular order.
Vikavolt
If you need an absurdly hard-hitting special attacker, Vikavolt gets the job done. With 145 base Special Attack and access to moves like Bug Buzz, Thunderbolt, and Energy Ball, it can dish out hits with ease. It's also available within the first few routes of the game and has the Levitate Ability -- further boons to its usability. Vikavolt is lacking in Speed and bulk though, so play carefully with it against fast special attackers.
Boltund
While it has a lower BST than some other Electric types, Boltund gets the job done as one of the few Electric types that can run a physical moveset. 90/90 offensive stats allow it to be customized to exploit any Pokemon's weaker side, with a serviceable movepool that includes Nuzzle, Wild Charge, Play Rough, Crunch, and Psychic Fangs. Its base 125 Speed enables it to strike fast and hard. However, like Vikavolt, it's quite frail. If it doesn't get the KO, it may be time for a Hyper Potion or Revive.
Toxtricity
Toxel comes just after the first Gym, where it can be received in the Nursery on Route 5. Use a few EXP Candies on it to bring it up to speed with the rest of the team, and it'll pay dividends in the long run. Though Toxel is fairly useless, Toxtricity at Level 20 comes with high Attack and Special Attack, along with moves like Overdrive, Sludge Bomb/Wave, and Boomburst, which pair well with its Punk Rock Ability. Like most Electric types, it's not very bulky and Ground types are particularly deadly -- but keep it in against opponents like Opal, and it'll perform brilliantly.
