I was unable to get the proxy to work, the complexity was too great due to my inexperience.
I also found that the initial migration option where there was a possibility of data loss too risky. A middle ground option could be an option to put the source server in read-only mode (eg. disabling IMAP/SMTP listeners and non-admin logins to JMAP), and a script that can transfer all users via Vandelay for example.