I have a backup device that pulls backups off my MIAB box so I did this to get at least one borg backup on the disk (untested)
My borg backup is twice as big as the old duplicity backup so I think the exclude isn’t actually working
# cd /home/user-data/backup
# mkdir borg
# cd borg
# export BORG_PASSCOMMAND="cat ../secret_key.txt"
# borg init --encryption keyfile .
# borg create --compression auto,zstd,22 --exclude 'backup/*' .::miab /home/user-data/