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.