> I’m running an 8-drive ZFS RAIDZ2 pool. I’m wondering if you know — are the free space recommendations around ZFS cargo or real?

I'm not entirely sure, but it seems to me that free space (and the 20% reservation) is mostly a proxy for fragmentation, and you can therefore better look at fragmentation directly. That would mean that if you mostly store large files, there shouldn't be a lot of fragmentation even at high utilization. The whole "ZFS changes allocation algorithm from 80% usage on" is something of 10+ years distant past, and lots of things around the allocator have been improved. It's also something that probably isn't too different from the performance of other filesystems at high utilization, so it shouldn't be exaggerated.