Following The Computer Backup Rule of Three I'm finally getting around creating my backup workflow. So far, what's working for me is:

• 3 copies of everything I care: two hard drives + B2 Cloud Storage for the last copy (currently spending $1 / month for 200 GB of data)
• 2 different formats (hard drive + cloud backup)
• 1 off-site backup (cloud backup)

But first I needed to de-duplicate my multiple (previous) backups. That's where the combo czkafka (discovered thanks to sebsauvage) + the venerable rsync come in play.

1. Copy everything to HDD no. 1
2. Ensure a copy of all the files is rsync'ed to HDD no. 2
3. Work through de-duplicates in HDD no. 1 using czkawka
4. Once satisfied, another rsync with --delete to sync the dedup to HDD no. 2
5. Upload final backup to B2 Cloud Storage

It helps that I archive documents and photos by year. So I can create a TAR file and upload, per year, to B2. I know no more content will be created for that year (well, until I can get back in time).