I also use backrest/restic on my NAS, but I went with a Hetzner StorageBox instead, a little cheaper for 1TB (I pay 5USD monthly including VAT, billed monthly too).
I still have to check if this actually works in practice, but I am hopeful. I based it off their documentation here: https://docs.hetzner.com/storage/object-storage/faq/s3-crede...
The main problem is that it sometimes slows down to a crawl, or requests fail altogether.