Beside checkbox "Skip Non-owned files" use a Filter option to filter out Shared With me folder names:

Even though "Skip Non-Owned files" vs Filter setting may look like identical functionality, in the background they use different algorithms for different operations. Combination of two avoid migration of any content from "Shared With Me" GDrive folder.
Note, when migrating only one account, User Mapping contains only one user, no need to map all the accounts.