to transfer pokemon from GO to Let's GO all you need to do is pair each trainer id/phone (in Pokemon GO app's -- Settings -> Nintendo Switch -> Connect) with the game (Open Pokemon GO settings)

once the account of the trainer is paired ... just start transfer as usual at Fuschia City's GO Park terminal

fastest way would be skipping some trainers along the route. its better you just take your time and enjoy going through the story otherwise its going to be a waste.

its like completing 60% of the story line before you can reach Fuchsia City.

Not possible.
You can’t even even copy your save file out to a pc.

not sure if it's the fastest way possible but i went around the only path accessible (some parts of map was not open at that point) around the right hand side of the map after the 3rd gym boss - Lt Surge.

from the walkthrough guides it seems the "usual" recommendation will be after 4th gym leader Erika. so i was around 35% into the story when i reached Fuschia City.
the game does not even support Cloud backup (let alone copying it out from a console) so it's a No.