It still sucks that S3 is ~$20/mo/TB basically "in perpetuity", while random SSD drives are ~$80/TB and I'd feel comfortable effectively amortizing them out at ~$20/yr for local storage instead of $20/mo for S3. :-/
ZeroFS will work with any S3 implementation that supports conditional writes. Even something a bit premium-priced like CloudFlare R2 goes for around $15/TB and has no egress costs. If you want to go for as cheap as possible, I'd throw minio or similar on a few cheap Hetzner storage servers https://www.hetzner.com/dedicated-rootserver/matrix-sx/