This topic has been going back and forth with various more or less related issues. Here I provide something like a conclusion of what I ended up doing.
- I gave up on Multcloud. It is just too buggy/unreliable and when it works, transfer is rather slow.
- I used the cheapest VPS from Scaleway to run rclone (their forum is at forum.rclone.org)
- Setting up rclone to use Amazon Cloud Drive is a bit of a pain, but thanks to these instructions, it is entirely possible.
- I ended up not migrating to B2 but to the German Amazon instead, since they still offer unlimited storage.
- Transferring from ACD (US) to ACD (DE) with this setup works extremely smoothly so far: 1 TB transferred within less than 12 hours.
Thanks to everyone helping me with this!