Well for me I like to keep it simple, I don’t do full image backups, I just backup the data. If you can get organized there are a couple programs that can help keep that data up-to-date on your backups as you contribute more to the pile.
If you wanted to attempt it I would say start with a large external hard drive, it would be a cheap setup. Start creating basic folders on the drive - pictures, music, videos, etc. Connect the external drive to each computer and go through it meticulously copying stuff over to the folders you have organized, don’t worry so much about duplicates at first, you are just trying to preserve the data.
Once you have everything copied over you can go back through it and get rid of the stuff that doesn’t work or is duplicated, you may even be able to identify this type of stuff easier now that its organized. After this process you should be confident enough that you could technically wipe all your computers and still have all of your data.
Identify one computer that you use the most, reserve that one computer for adding new data to your collection. You want to be able to copy all the folders from your external hard drive, so that you have the same data in both locations.
Once you have the data in both locations, download a program called freefilesync, this programs works on Windows, Mac, and Linux; so if you ever decide to switch OS’s you are covered. Setup the program to duplicate changes that you make on your computer to the external hard drive.
Now you have a workable onsite backup in place that is easy to manage, you simply keep your files organized on you computer as you add them, and every once in a while, launch freefilesync (make sure your external hard drive is connected and turned on), and click the sync button to sync all the files to your backup.
Now if you decide to do offsite backups, which is a good idea in case of a major disaster, you can easily setup something like google drive and sync all of your (already organized) folders to your google account. Again, you can launch the program whenever you want to sync new files.
I would recommend keeping the external hard drive off, and prevent google drive from launching at startup, just so that you have control over when the syncs occur in case your computer gets infected. You don’t want anything bad to spread to your external hard drive or offsite backup.
There are certainly other ways to do backups better than my system, where you can have incremental backups and cherry pick the backup you want to restore, but those may have higher costs.