DID YOU USE THE ZFS EXPERIMENTAL INSTALL? That would explain your memory usage, since ZFS reserves a large part of the free memory as cache and that is counted as used memory. It will free that memory again, if you load more programs, but that makes your system somewhat slower. I use ZFS since March 2018 and I boot from ZFS since Jan 2019. I maximize the cache size to 20-25% of my total memory in /sys/module/zfs/parameters/zfs_arc_max. By default it contains 0, no limit, but you can specify there a large number, specifying the maximum size of the cache in BYTES.
You can type: "arc_summary" to get a lists of all ZFS sizes and performances. It is a huge list, but in the first 10 lines you can see the cache (L1ARC) size. Somewhat further on it will give you even the cache hit rates.
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