I've found restic + rclone to be extremely stable and reliable for this same sort of differential backup. I backup to Backblaze B2 and have also used Google Drive with success, even for 1TB+ of data.
I agree. restic with it's simple ability to mount (using fusefs) a backup so you can just copy out the file(s) you need, is so wonderful. A single binary that you just download and can SCP around the place to any device etc.
It's fantastic to have so many great open source backup solutions. I investigated many and settled on restic. It still brings me joy to actually use it, it's so simple and hassle free.
Kopia supports zstd compression, I found it to be within +10% to -5% of the size of my Borg repo.
It also has extensive support for ignoring stuff and it works very well.
I still use Borg because its policy of expiring older snapshots is more useful for me, but Kopia is extremely solid and I would use it any day if I didn't care that it doesn't actually keep one monthly backup for the last 3 months as Borg does (it decides which older snapshots to keep with another algorithm; it's documented on their website).
