Mac OSX file / folder sizes vs web file / folder sizes

I am using iCloud for these features:

• I want some files available on my iPhone / iPad / to share with friends.
• This is a small number of files / small total size (less than 20GB).
• Office Online works with iCloud (so free Excel for your iPad for instance).
• Pricing: I bought 200GB initially ($3 per month) but I will go down to their lowest plan (50GB, $1 per month). I can change it anytime with no contracts. So less than $12 per year for that functionality for me.

iCloud is NOT a backup service. It is a sync service which allows all your devices to edit and view files stored on your computer. If you delete a file from any device, it is deleted from iCloud. Other than manual drag and drop there is no official interface to back those files up.

If you choose to minimize local space, the only copy is in iCloud. Some files on your local drive would just be links, so Time Machine can't back them up unless you do NOT choose this option and instead store complete copies locally. That requires a lot of space which isn't a good solution if you have many / large files. In your case with 4 computers, you already have space reserved for these files so very little changes... make sure to stop the Dropbox sync before migrating!

I am using rsync.net for cloud backups. I am very familiar with their service, having researched and implemented it at a previous job for daily backups for dozens of servers. Today I bought 500GB of space (their smallest option) for $108 for a year. This gives me many features I did not have with Dropbox, for instance:

• It gives me daily backups for x days, then weekly backups for y weeks, then monthly backups for z months, with optional quarterly / yearly backups also. All configurable, and you're paying for the storage used not the frequency. 100GB of files with 10GB of changes each day for a week is 170GB of storage plus some overhead. NOT 100GB * 7.
• Backups are automatically rotated. There is zero maintenance for me. I am warned when I get near my total, and there is 10% extra allocated "just in case".
• Accessible via rsync with or without ssh certificate(s), or FTP from anywhere.
• All backups are read-only and cannot be accidentally deleted or modified. This is pretty huge, mistakes happen under pressure.
• You have a "live" set of files that you sync to. When it's time for a "backup", the system automatically generates a new folder named "weekly-2019-09-22" or similar but no actual files have to be copied. So it's instant! You refer to anything not live by opening one of these folders.
• They scale to petabytes. (It's a ZFS snapshot, similar to an LVM snapshot).
• In my use case, I will have a "backups" folder, and also several top-level "software" and other folders which I will move to an external drive locally and sync to the server on an automated schedule. This is similar to DropBox's option to not sync all folders but because it doesn't have to be located in the "main" sync folder, I can make a separate job for it and run it separately or in parallel if I like.
• rsync allows me to limit bandwidth per-job, a feature not available in other systems such as Dropbox.
• rsync allows me to save Unix permissions and ownership, again not available in Dropbox or other systems.
• rsync is super fast -- hundreds of thousands of files can be compared in a few seconds. Once the initial sync is complete, future synchronizations are as quick as possible, featuring block-level comparison and compression.
• I can customize a list of files/folders/volumes to ignore for each job, or have each job use the same list.
• Look up the rsync protocol to see features. It's mature and open-source and pretty much every UNIX/Linux variant has it.

The iCloud piece is simple, anyone can set it up. The rsync piece is significantly more technical. Unless you are comfortable using the command line and can follow their instructions to setup a SSH certificate, I would recommend you use a different backup solution.

Windows was very slow with rsync in my experience due to Cygwin limitations, so if you only had Windows, I would say find another solution. I had Linux servers in the same location as the Windows servers. I created Windows shares for everything I wanted to rsync, mounted them in Linux, and used the native rsync Linux software, which ran at full speed, at least 4-10x faster than Windows.