This may be partially due to the drives themselves.
Can you please post the output of sudo fdisk -l so we can see the drive details and check the data sheets.
If the drives happen to be the SMR (shingled magnetic recording) tech then slowdown is a prevalent fact for those devices. The older and still relevant CMR tech used for datacenter and NAS drives does not have the same problems with write speeds.
Drives with SMR tech begin to show major degradation in write speed once they have about 1/4 of the advertised storage space in use