How can I transfer my save data from a friend's Switch to my new one?

I borrowed a Switch from a friend to see if I wanted to buy one. Now I have enough to get one.

Since I’ll be setting up my own Nintendo account, how do I get my save data from that switch to the new one without using his account? I’m almost done with the main story mode, which means I can start catching shiny and high-IV Pokémon, and I don’t want to start all over.

Individual save data transfer is quite difficult on the Switch. I believe you can transfer an entire Switch's configuration, but not individual game save data. (Unofficially, there are complicated ways to transfer individual save data through homebrew file managers.)