I'm not an expert, but I just gave a similar answer to someone else in a similar situation (significantly older dovecot version on old server) and I suggest trying to use doveadm backup on the new server to pull the mail from the old server via IMAP like:
doveadm -o imapc_features=fetch-headers -o mail_prefetch_count=20 -o imapc_port=993 -o imapc_ssl=imaps -o imapc_ssl_verify=no -o mail_fsync=never -o imapc_host=mail.example.com -o [email protected] -o imapc_password=123456 backup -R imapc:
