YouTube Music limitations
If you're experiencing the issue associated with transferring a lot of playlists to YTM at once, then, unfortunately, there is a limit on how many playlists can be added at the same time. As a workaround and to overcome the issue, you can wait up to 3 hours until you set up the transfer for another few playlists.
Hence, it seems that at the moment you can add 5-10 playlists at once every 3 hours.
We are truly sorry for this inconvenience, nevertheless, at the moment we are unable to skip it due to the terms and regulations of the YouTube Music service.
If you're experiencing any other issues or have some questions, reach out to us and firstname.lastname@example.org :)