i.e. it has nothing to do with being on an SSD. It is due to the technical limitations of NTFS.

The main driver would be the file system in use.

HFS+ (most likely on El Capitan) was designed to resist defragmentation on old skool drives. APFS (High Sierra) OTOH, seems to have issues when running on spinning rust v. SSD.

Upgrades are in place with on the fly conversion of HFS -> HFS+ -> APFS (High Sierra only). The observed cloud activity was likely the update manager pulling down the latest security packages. Unless you have a trivial amount of data, uploading an entire disk would take a very noticeable time on the average residential broadband connection.

Defrag-wise, HFS+ on SSD doesn't need it. On HDD, very large files (like virtual machine guest image) may suffer and need rearranging (but it may be simpler to keep an unadulterated copy and just clone the VM when it becomes unwieldy.)

If you're worried about wear leveling, take a look at disabling sleep & suspend. The default setting dumps the entire memory image to disk every time you close the lid.