You’re not correct on a number of fronts here.

ZFS is not tied to the memory allocator in Solaris, I’m very unclear how you could have made that mistake.

128bits might feel wasteful, but you’re likely never going to have the problems FAT32 has had- it’s designed for the next 100 years- which has drawbacks but prevent the kinds of ugly hacks that NTFS has gone through to maintain forward compatibility (rolling on disk upgrade).

And yes, the 1G per TB is for the in memory block map, which is a requirement for deduplucation on the default block size.