Density is exactly the problem. The chances of an unrecoverable error are around 1 per trillion! But oh wait, you have 2 trillion bits on that disk?

I've been running ZFS on several servers with tenfolds of TB's with data. I see checksum errors every month, from a single bit to several megabytes.

That sounds like you have something else wrong. With ECC memory on server hardware, we've seen 0 checksum errors in the last 6 months and I've seen only 2 ever. A typical server has 136TB of raw hdd space and we get about 71TiB usable. It's about 80% full.