Pretty much the second answer of MythicalData:
You need an unpatched Switch which is still unbanned. Theoretically you only need it "for a moment" to get the job done so a friend's Switch should be fine, you don't even need to buy another one.
Then log into your account, download/install/insert the game, make sure the cloud backup of the save is downloaded to your console and then back it up with Checkpoint/EdiZon (maybe risky) or HacDiskMount (not risky).