Grab small servers with one or two big disk (Hetzner will give you two 4TB disks for 39.00 EUR, OVH one 6TB for about 30 USD w/ an ARM CPU, less than 50 USD for

Grab small servers with one or two big disk (Hetzner will give you two 4TB disks for 39.00 EUR, OVH one 6TB for about 30 USD w/ an ARM CPU, less than 50 USD for two 4TB under the soyoustart brand), serve them over iSCSI, use RAID-Z3 to create a nice huge ZFS filesystem. Do not forget to have hot spares. Done. Costs about half or even third of what S3 costs and it's a filesystem instead of an object storage. Also has transparent compression. ZFS grows online. Both OVH and Hetzner and I bet other such providers have API so you can just script the whole thing to grow as soon as you cross a threshold.