In regards to the second point, this isn't correct for ZFS: "If several files contain the same pieces (blocks) of data or any other pool data occurs more than once in the pool, ZFS stores just one copy of it. Instead of storing many copies of a book it stores one copy and an arbitrary number of pointers to that one copy." [0]. So changing one byte of a large file will not suddenly result in writing the whole file to disk again.

[0] https://www.truenas.com/docs/references/zfsdeduplication/