I have managed to copy with rsync and getting 180 MB/s. I guess my initial assumption was wrong, HDD is obviously not bottleneck here, it can get close to ISP speed. Thank you for pointing this out, Ill do more testing these days. Im kinda shocked because I never knew HDD can be that fast. Gonna reread all the comments as well
Ill look into ZFS, but in meantime I found out my HDD is probably not bottleneck. Still want to learn about this so thanks for your comment