The field of author services (as actual paid-for services, not bundled with journal publication) is an interesting field. Peerwith [1] is one new entrant into the field, which is part of the Springer-Nature incubator Digital Science. They're sort of a marketplace for academics to connect with (and pay) other academics to help with manuscript services, everything from peer review type work to copy editing.

[1] https://www.peerwith.com/