Backblaze and similar services not necessary for a small hdd? Upload full disk dmg to cloud services easier?
Firstly, I think this depends on your personal price sensitivity, and factors like desire, or expectation for single file restoration, and tracking deleted files, as well as your internet connection. This is a fundamentally subjective and opinion based question.
Yes, you could image and save the whole image every month, but that comes with several drawbacks:

Backblaze is 'only' $4 more per month - this is dependent on your price sensitivity.
You have to remember to do it (although this could probably be automated, that's an investment of your time and energy). Something like CarbonCopyCloner will give you drive image back-ups.
You only get monthly point-in-time restoration. If your drive dies tomorrow (or you lose local back-ups), you're losing a month of data.
Backblaze gives you the ability to restore files from anywhere.
Backblaze gives you the ability to restore individual files quickly and easily
Uploading a whole drive every month could be taxing / time consuming on your internet connection.

To directly answer the questions in the title:

Necessary - this is entirely dependent on your risk tolerance and highly subjective. My personal opinion - backup everything that's personal to you (ie. can't be replaced), regardless of size.
Easier, no, Backblaze you set and forget and it uploads files every time they change. It's about as easy a solution as you can possibly get.

Backblaze is not your only option though:

iCloud will give you all your Documents off-site, and integrates into iOS and macOS so that it's as easy as Backblaze
Something like Arq allows you to backup to many different cloud providers - eg. AWS S3, and include Google Drive as a target option.